The price of MSTX will be influenced more by soon to be released data on its Phase III study. According to a report from BIO (Clinical Development Success rates 2006-2015 pg 7), there's a 58% chance of Phase III data leading to an NDA and once an NDA is submitted a 85.3% chance of approval.
There is a lot of potential for a huge swing either direction. Personally, I think it's a buy, I have confidence in the leadership and there aren't many options for treating sickle-cell. Don't be the entire bank though, it's still about the science.