Despite having only two exits under its belt since inception in 2007, Third Rock Ventures closed its third fund last week with $516 million. The fund, the firm's largest to date, was oversubscribed and raised in about five months.

Third Rock's Kevin Starr said the firm started fundraising with a target of $425 million. There was a "very high level of receptivity from investors," he said. "Investors found something that was highly differentiated and gives them good odds for high returns over time."