Citi analyst Robyn Karnauskas upgraded Biogen (BIIB) to Buy with an increased price target of $305. The stock closed yesterday up 24c to $264.91.

With the Alzheimer's Phase 2 data already reported and no Phase 3 results until 2019/20, upside in the stock rests on the stability of Biogen's multiple sclerosis business and success of the Spinraza launch in spinal muscular atrophy, Karnauskas tells investors in a research note.

Her firm's proprietary survey suggests upside to Spinraza estimates. Further, the analyst's "deep dive" analysis suggests longer term estimates for Biogen's MS business are fair to conservative.