Elan Corp. plc (NYSE:ELN) investors did an about-face last Friday after the biotech said it would return to shareholders $1 billion of the $3.3 billion in proceeds from its restructured deal for Tysabri natalizumab. The news sent Elan shares up $0.31 to $10.60 and a market cap of $6.3 billion.

The move translates into a $700 million market cap bump