If it is a bubble, then what investors most keenly want to know is, when is it likely to burst. "I don't view it as a bubble," Drone said. "Industries go in and out of favor," he added, noting that the phenomenon of large full-service pharmaceutical companies facing holes in their drug pipelines from loss of exclusivity could give the biotechs quite a long run. "If a biotech company is attractive, then yes, there's going to be a significant positive upward push on its share price. The large pharma with an established commercial infrastructure can take that asset, even at the higher price, and commercialize it, so it's not that expensive to them. Mendelson also said that while he doesn't "think it's unlimited," higher valuations probably won't slow down within the next three to four years. Perhaps the high IPO rate for biotechs and heightened valuations won't end for a year, or more, but investors should definitely keep their eye on the marketplace. As Schoenebaum said, "Think about what it means when a platform technology company is oversubscribed and trades up 50% on its first day. These are the kinds of things that happen when you look back historically and say, hey, that was a bubble. ... We are in a very, very, special time that raises some questions about when all this ends, and will it end badly or not?"