Ebb & Flow

Nobody disputes the notion that Elan's fortunes are more closely tied to multiple sclerosis drug Tysabri natalizumab than partner Biogen Idec's. Indeed, Tysabri news - be it good or bad - has typically led ELN's share price to rise or fall more sharply than BIIB's. But last week, ELN seemed to be punished disproportionately after FDA approved the reintroduction of the antibody, which had been pulled from the market in February 2005 because its use was associated with progressive multifocal leukoencephalopathy (PML).

On the week, ELN plunged $3.01 (16%) to $15.97 while BIIB fell $1.71 to $46. BIIB's decline mirrored the 4% drop on the NASDAQ...