1 [BIOGEN, INC. LETTERHEAD] EXHIBIT 10.52 PERSONAL AND CONFIDENTIAL January 7, 1999 Mr. James C. Mullen Biogen, Inc. 14 Cambridge Center Cambridge, MA 02142 Dear Jim: This confirms your discussion with Jim Vincent related to your new role in the organization. Effective January 1, 1999, your new title is President and Chief Operating Officer with a base salary of $400,000 and a target bonus of 60%. Additionally, you have been granted 175,000 stock options as of January 4, 1999 with a grant value of $14,525,000 (175,000 shares at $83.00). Sincerely, /s/ Frank A. Burke, Jr. Frank A. Burke, Jr. Vice President, Human Resources cc: Personnel File