The 2015 return of 12.3% for the NASDAQ Biotechnology Index ETF makes the biotech sector one of the market’s best-performing baskets this year, and the returns of the five best biotech stocks in the space were downright mind-boggling. All five of these biotech companies boast billion-dollar market caps, and each has seen its shares post eye-popping returns of nearly 200% or more this year. Let’s find out why.