When is $100 million not $100 million? When it’s a proxy, maybe even something akin to a bet.The nice, round figure is the target initial public offering value for Editas Medicine, a biotechnology firm with a mission of using gene editing to treat disease. The company has already raised more than $160 million from investors, and initial evaluations of the IPO said it was almost certain to be exceeded when the stock started trading publicly.But Editas could face a big problem.