X0306 4 2021-04-01 0 0001280600 ACCELERON PHARMA INC XLRN 0001639637 Smith Karen L. 128 SIDNEY STREET CAMBRIDGE MA 02139 1 0 0 0 Common Stock 2021-04-01 4 M 0 5000 35.34 A 9561 D Common Stock 2021-04-01 4 M 0 1875 41.64 A 11436 D Common Stock 2021-04-01 4 M 0 2050 52.99 A 13486 D Common Stock 2021-04-01 4 S 0 1201 134.54 D 12285 D Common Stock 2021-04-01 4 S 0 2987 136.22 D 9298 D Common Stock 2021-04-01 4 S 0 3796 136.92 D 5502 D Common Stock 2021-04-01 4 S 0 941 137.92 D 4561 D Option to Purchase Common Stock 35.34 2021-04-01 4 M 0 5000 0 D 2027-11-15 Common Stock 5000 15000 D Option to Purchase Common Stock 41.64 2021-04-01 4 M 0 1875 0 D 2029-01-25 Common Stock 1875 5625 D Option to Purchase Common Stock 52.99 2021-04-01 4 M 0 2050 0 D 2030-01-22 Common Stock 2050 6135 D The reported transaction was effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $134.10 to $135.05 inclusive. The reporting person undertakes to provide Acceleron Pharma Inc., any security holder of Acceleron Pharma Inc., or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (2) to this Form 4. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $135.72 to $136.64 inclusive. The reporting person undertakes to provide Acceleron Pharma Inc., any security holder of Acceleron Pharma Inc., or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (3) to this Form 4. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $136.74 to $137.36 inclusive. The reporting person undertakes to provide Acceleron Pharma Inc., any security holder of Acceleron Pharma Inc., or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (4) to this Form 4. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $137.77 to $138.16 inclusive. The reporting person undertakes to provide Acceleron Pharma Inc., any security holder of Acceleron Pharma Inc., or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (5) to this Form 4. The shares of common stock underlying this stock option award vested in equal quarterly installments over the first three years after the grant. The shares of common stock underlying this stock option award vested in full on the one year anniversary of the grant date. /s/ Adam M. Veness, as attorney-in-fact for Karen L. Smith 2021-04-02