POWER OF ATTORNEY

        KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below hereby revokes all powers of attorney relating to the following matters
and constitutes and appoints Mark W. Leuchtenberger, George A. Eldridge, Robert
Solomon, Lee S. Feldman, Jolie M. Siegel, Matthew M. Graber, and Eric Y. Wu, and
any one of them acting singly, the true and lawful attorneys-in-fact and agents,
with full power of substitution and resubstitution, for the undersigned and in
the undersigned's name, place and stead, in any and all capacities (until
revoked in writing) to sign any and all instruments, certificates and documents
required to be executed on behalf of the undersigned as an individual or on
behalf of the undersigned's holding company, as the case may be, pursuant to
sections 13 and 16 of the Securities Exchange Act of 1934, as amended (the
"Exchange Act"), and any and all regulations promulgated thereunder, and to file
the same, with all exhibits thereto, and any other documents in connection
therewith, with the Securities and Exchange Commission, and with any other
entity when and if such is mandated by the Exchange Act or by the By-laws of the
Financial Industry Regulatory Authority, granting unto said attorneys-in-fact
and agents full power and authority to do and perform each and every act and
thing requisite and necessary fully to all intents and purposes as the
undersigned might or could do in person thereby ratifying and confirming all
that said attorneys-in-fact and agents, or their substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

        IN WITNESS WHEREOF, this Power of Attorney has been signed as of October
8, 2007.

                                        Signature:    /s/ Robin Steele
                                                      --------------------------
                                        Name:         Robin Steele
                                        (please print)