On Change of Control (either ELN or BIIB) the company not being acquired has the right to buy out the other's half of Tysabri. It would be subject to valuation, of course.
Tysabri’s annualized worldwide sales are roughly $900M (#msg-37091385). Although robust growth could conceivably resume, it’s also possible that Tysabri sales have reached a permanent plateau. Based on the $900M figure, a valuation of 5x sales would make ELN’s 50% stake worth about $2.25B.
If ELN owned Tysabri outright, a somewhat higher valuation, say 6x sales, would be warranted because the costs of maintaining a JV infrastructure could be eliminated. This would put the valuation in the ballpark of your $5B figure.
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