Exhibit 31.3
I, James C. Mullen, certify that:
1. I have reviewed this annual report on Form 10-K/A of Biogen Idec Inc.; and
2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report.
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| | By: | /s/James C. Mullen | |
| | | James C. Mullen | |
| | | Chief Executive Officer and President | |
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Date: April 29, 2008
