EXHIBIT 99.12c


INTERNAL REVENUE SERVICE                  Department of Treasury
                                          Washington, DC 20224

Index Number: 368.03-00, 368.03-01,       Third Party Communication: None
              368.13-00                   Date of Communication: Not Applicable

                                          Person To Contact:
Mr. Jon Zindel                            Peter C. Meisel, ID No. 50-28834
Tax Director
American Century Investments              Telephone Number:
4500 Main Street                          (202) 622-7750
Suite 400
Kansas City, MO 64111                     Refer Reply To:
                                          CC:CORP:1
                                          PLR-152157-05

                                          Date: February 10, 2006




LEGEND:

Acquiring Fund    =    American Century Mutual Funds, Inc. - Select Fund,
                       a Maryland corporation
                       EIN 43-1607636

Acquiring Series  =    American Century Mutual Funds, Inc.

Target Fund       =    Mason Street Funds, Inc. - Growth Stock Fund,
                       a Maryland corporation
                       EIN 39-1861707

Target Series     =    Mason Street Funds, Inc.

State A           =    Maryland

X                 =    30

Y                 =    84.29

Shareholder C     =    The Northwestern Mutual Life Insurance Company and
                       Maroon, Inc.



Dear Mr. Zindel:

This letter responds to your request dated October 11, 2005, for rulings as to
the federal income tax consequences of a proposed transaction. Additional
information was submitted in letters dated November 14, 2005, December 21, 2005,
January 6, 2006,




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January 27, 2006, February 6, 2006, and February 9, 2006. The information
submitted is summarized below.

Acquiring Fund is a State A corporation registered under the Investment Company
Act of 1940 (the "1940 Act") as a diversified, open-end management investment
company. Acquiring Fund is a series of Acquiring Series. As a series fund,
Acquiring Fund is treated as a separate corporation and separate taxpayer for
federal income tax purposes pursuant to § 851 of the Internal Revenue Code
(the "Code"), and has elected to be treated as a regulated investment company (a
"RIC") under §§ 851 through 855.

Target Fund is a State A corporation registered under the 1940 Act as a
diversified, open-end management investment company. Target Fund is a series of
Target Series. As a series fund, Target Fund is treated as a separate
corporation and separate taxpayer for federal income tax purposes pursuant to
§ 851, and has elected to be treated as a RIC under §§ 851 through 855.

Pursuant to a plan of reorganization, it is proposed that Acquiring Fund and
Target Fund will undertake the following transaction (the "Reorganization"):

     (i)  Target Fund will transfer substantially all of its assets and
          liabilities to Acquiring Fund in exchange for shares of Acquiring Fund
          having equivalent value to the net assets transferred;

    (ii)  Target Fund will distribute pro rata the shares of Acquiring Fund
          received in step (i) to the shareholders of record of Target Fund in
          redemption of all shares of Target Fund; and

   (iii)  Target Fund will liquidate and dissolve in accordance with the laws of
          State A, and terminate its registration under the 1940 Act.

Within X days before the Reorganization, pursuant to § 22(e) of the 1940 Act,
it is proposed that a shareholder and its affiliate (together, "Shareholder C")
holding Y% (which is an amount greater than 50%) of the stock of Target Fund
will present for redemption, solely for cash, shares representing up to 10% of
the net asset value of the Target Fund as of that date (the "Redemption"). Other
than the Redemption, Shareholder C has no plan or intention to present any other
shares for redemption in connection with the Reorganization. Shareholder C is an
affiliate of the investment advisor of Target Fund. Under § 22(e) of the 1940
Act, each shareholder of an open-end management investment company has the right
to redeem any or all of its interest in the investment company on any business
day of the year.

The following representations have been made in connection with the
Reorganization:

     (a)  The fair market value of the shares of Acquiring Fund that will be
          received by each Target Fund shareholder will be approximately equal
          to the fair market




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          value of the shares of Target Fund that will be surrendered in
          exchange therefor. In the Reorganization, the Acquiring Fund will
          issue no consideration to the Target Fund shareholders other than
          Acquiring Fund shares (including fractional shares, if any) in
          exchange for their Target Fund shares.


     (b)  On the date of Reorganization and at all times after, there will be no
          plan or intention by Acquiring Fund or any person related to Acquiring
          Fund (as defined in Treas. Reg. § 1.368-1(e)(3)) to acquire or redeem
          any of the Acquiring Fund shares issued in the Reorganization either
          directly or through any transaction, agreement, or other arrangement
          with any other person, other than redemptions that Acquiring Fund will
          make as an open-end investment company pursuant to § 22(e) of the
          1940 Act.

     (c)  During the five year period ending on the date of the Reorganization,
          neither Target Fund nor any person related to Target Fund (as defined
          in § 1.368-1(e)(3) without regard to § 1.368-1(e)(3)(i)(A)) will
          have (i) acquired Target Fund shares with consideration other than
          shares of Acquiring Fund or Target Fund, except for shares redeemed
          pursuant to the Redemption or in the ordinary course of Target Fund's
          business as an open-end investment company pursuant to § 22(e) of
          the 1940 Act, or (ii) made distributions with respect to the Target
          Fund shares except for (a) normal, regular, dividend distributions
          made pursuant to the historic dividend paying practice of Target Fund,
          and (b) distributions and dividends declared and paid in order to
          ensure Target Fund's continuing qualification as a RIC and to avoid
          the imposition of fund-level tax.

     (d)  Prior to or in the Reorganization, neither Acquiring Fund nor any
          person related to Acquiring Fund (as defined in § 1.368-1(e)(3)) will
          have acquired, directly or through any transaction, agreement or
          arrangement with any other person, Target Fund shares with
          consideration other than Acquiring Fund shares.

     (e)  Acquiring Fund will acquire at least 90 percent of the fair market
          value of the net assets and at least 70 percent of the fair market
          value of the gross assets held by Target Fund immediately prior to the
          Reorganization. For purposes of this representation, amounts used by
          Target Fund to pay its reorganization expenses and all redemptions and
          distributions made by Target Fund immediately preceding the transfer
          (except for (i) redemptions of shares pursuant to § 22(e) of the
          1940 Act (other than the Redemption) and (ii) distributions and
          dividends declared and paid in order to ensure Target Fund's
          continuing qualification as a RIC and to avoid the imposition of
          fund-level tax) will be included as assets of Target Fund held
          immediately prior to the Reorganization.

          In making this representation, the Redemption shall be taken into
          account by treating the Target Fund as if it distributed, immediately
          prior to the




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          Reorganization, a percentage of its net and gross assets,
          respectively, equal to the actual percentage of the Target Fund's net
          and gross assets distributed to Shareholder C on the date of the
          Redemption. For purposes of the preceding sentence, the Target Fund
          shall be treated, immediately prior to the deemed distribution above,
          as if it held an amount of net and gross assets that bears the same
          proportion to its actual net and gross assets (at that time) as the
          amount of the Target Fund's net and gross assets immediately before
          the Redemption bore to its net and gross assets immediately after the
          Redemption.

     (f)  The liabilities of Target Fund that will be assumed by Acquiring Fund
          (within the meaning of § 357(d)) were incurred by Target Fund in the
          ordinary course of business and are associated with the assets
          transferred to Acquiring Fund.

     (g)  Acquiring Fund is in the same line of business that Target Fund was in
          preceding the Reorganization for purposes of § 1.368-1(d)(2).
          Following the Reorganization, Acquiring Fund will continue such line
          of business and has no plan or intention to change such line of
          business. Neither Acquiring Fund nor Target Fund entered into such
          line of business as part of the plan of reorganization. On the date of
          Reorganization, at least 33 1/3% of Target's portfolio assets will
          meet the investment objectives, strategies, policies, risks and
          restrictions of Acquiring Fund. Target Fund did not alter its
          portfolio in connection with the Reorganization to meet the 33 1/3%
          threshold. On the date of Reorganization, Acquiring Fund will have no
          plan or intention to change any of its investment objectives,
          strategies, policies, risks and restrictions after the Reorganization.
          To the best of the knowledge of the Acquiring Fund's management, as of
          the record date for Target Fund shareholders entitled to vote on the
          Reorganization, there was no plan or intention by the Target Fund
          shareholders to sell, exchange, or otherwise dispose of a number of
          Target Fund shares (or Acquiring Fund shares received in the
          Reorganization), in connection with the Reorganization, that would
          reduce the Target Fund shareholders' ownership of Target Fund shares
          (or equivalent Acquiring Fund shares) to a number of shares that was
          less than 50% of the number of Target Fund shares as of the record
          date.

     (h)  Target Fund will distribute all of the Acquiring Fund shares received
          by it in the Reorganization to its respective shareholders in complete
          liquidation in proportion to the number of Target Fund shares owned by
          each shareholder.

     (i)  Acquiring Fund, Target Fund, and the shareholders of Target Fund will
          pay their respective expenses, if any, incurred in connection with the
          Reorganization. If Acquiring Fund or the investment advisor to Target
          Fund pays or assumes expenses of Target




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          Fund, they will pay or assume only those expenses of Target Fund that
          are solely and directly related to the Reorganization in accordance
          with the guidelines established in Rev. Rul. 73-54, 1973-1 C.B. 187.

     (j)  At the time of the Reorganization, there will be no intercorporate
          indebtedness existing between Acquiring Fund and Target Fund that was
          issued, acquired, or settled at a discount.

     (k)  Acquiring Fund and Target Fund have elected to be taxed as RICs under
          § 851, and for all of their taxable periods (including the last
          short taxable period ending on the date of Reorganization for Target
          Fund), have qualified or intend to qualify for the special tax
          treatment afforded to RICs under the Code. After the Reorganization,
          Acquiring Fund intends to continue to so qualify.

     (l)  Acquiring Fund will not own, directly or indirectly; nor will it have
          owned during the five years preceding the date of Reorganization,
          directly or indirectly, any Target Fund shares.

     (m)  On the date of Reorganization, Target Fund will not be under the
          jurisdiction of a court in a Title 11 or similar case within the
          meaning of § 368(a)(3)(A).

Based solely on the information submitted and the representations set forth
above, we hold as follows:

     (1)  The Reorganization will qualify as a "reorganization" within the
          meaning of § 368(a)(1)(C). Acquiring Fund and Target Fund are each
          "a party to a reorganization" within the meaning of § 368(b).

     (2)  No gain or loss will be recognized by Target Fund upon the transfer of
          all of its assets to Acquiring Fund in exchange for Acquiring Fund
          stock and the assumption by Acquiring Fund of Target Fund liabilities
          (§§ 361 (a) and 357(a)).

     (3)  No gain or loss will be recognized by Target Fund on the distribution
          of Acquiring Fund stock to its shareholders (§ 361(c)).

     (4)  No gain or loss will be recognized by Acquiring Fund upon the receipt
          of the assets of Target Fund in exchange for Acquiring Fund stock (§
          1032(a)).

     (5)  The basis of the assets of Target Fund in the hands of Acquiring Fund
          will be the same as the basis of those assets in the hands of Target
          Fund immediately prior to the transfer (§ 362(b)).

     (6)  The holding period of the assets of Target Fund in the hands of
          Acquiring Fund will include the period during which those assets were
          held by Target Fund (§ 1223(2)).




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                                       6


     (7)  The basis of the shares of Acquiring Fund received by Target Fund
          shareholders other than Shareholder C will be the same as the basis of
          the Target Fund stock surrendered in exchange therefor (§
          358(a)(1)). The basis of the shares of Acquiring Fund received by
          Shareholder C will be the same as the basis of the Target Fund stock
          surrendered in exchange therefor, decreased by the amount of money and
          other property received by Shareholder C and increased by the amount,
          if any, which is treated as a dividend, and the amount of gain (not
          including any portion of such gain which is treated as a dividend, if
          any) Shareholder C may recognize on such exchange (§ 358(a)(1)).

     (8)  The holding period of Acquiring Fund stock received by the Target Fund
          shareholders will include the period during which the Target Fund
          shareholders held the Target Fund stock surrendered in exchange
          therefor, provided the Target Fund stock was held as a capital asset
          on the date of the exchange (§ 1223(1)).

     (9)  No gain or loss will be recognized by the Target Fund shareholders
          other than Shareholder C on the receipt of Acquiring Fund stock solely
          in exchange for their Target Fund stock (§ 354(a)). Shareholder C
          will recognize gain upon the receipt of Acquiring Fund stock and cash
          or other property in exchange for Target stock, but in an amount not
          in excess of the cash and other property received (§ 356(a)(1)). If
          the exchange has the effect of a distribution of a dividend
          (determined with the application of § 318(a)), then the amount of
          the gain recognized that is not in excess of Shareholder C's ratable
          share of the undistributed earnings and profits will be treated as a
          dividend (§ 356(a)(2)). The determination of whether the exchange
          has the effect of a distribution of a dividend will be made in
          accordance with the principles set forth in Commissioner v. Clark, 489
          U.S. 726 (1989). No loss will be recognized by Shareholder C pursuant
          to § 356(c).

     (10) Pursuant to § 381 (a) and § 1.381(a)-1, Acquiring Fund will
          succeed to and take into account the items of Target Fund described in
          § 381(c), subject to the provisions and limitations specified in
          §§ 381, 382, 383, and 384, and the regulations thereunder.
          Pursuant to § 1.381(b)-1, the tax year of Target Fund will end on
          the effective date of the Reorganization.

Except as expressly provided herein, no opinion is expressed or implied
concerning the tax consequences of any aspect of any transaction or item
discussed or referenced in this letter.

This ruling is directed only to the taxpayer requesting it. Section 6110(k)(3)
of the Code provides that it may not be used or cited as precedent.




PLR-152157-05
                                       7


In accordance with the Power of Attorney on file with this office, a copy of
this letter is being sent to your authorized representatives.

A copy of this letter must be attached to any income tax return to which it is
relevant. Alternatively, taxpayers filing their returns electronically may
satisfy this requirement by attaching a statement to their return that provides
the date and control number of the letter ruling.

The rulings contained in this letter are based upon information and
representations submitted by the taxpayer and accompanied by a penalty of
perjury statement executed by an appropriate party. While this office has not
verified any of the material submitted in support of the request for rulings, it
is subject to verification on examination.


                                 Sincerely,


                                 /s/ Michael J. Wilder
                                 ------------------------------------
                                 Michael J. Wilder
                                 Senior Technician Reviewer, Branch 1
                                 Office of Associate Chief Counsel (Corporate)




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                                       8


cc:      Karen Gilbreath Sowell
         Ernst & Young LLP
         1225 Connecticut Avenue, NW
         Washington, D.C. 20036

         Ronald S. Cohn
         Ernst & Young LLP
         560 Mission Street
         San Francisco, CA 94105

         Internal Revenue Service
         Attention: Industry Director, Financial Services (LM:F)
         290 Broadway, 12th Floor
         New York, NY 10007