Carl Icahn has hired Harvard geneticist Richard Mulligan as a portfolio manager, signaling an increased focus from the billionaire activist investor on an industry he's largely ignored for several years.

Mulligan is no stranger to Icahn. He worked with the billionaire on multiple biotech targets, from ImClone Systems — sold to Eli Lilly in 2008 for $6.5 billion — to Biogen, where Mulligan is still a director.

Mulligan worked closely with former Icahn deputy Alex Denner, who left to form Sarissa Capital in 2012. Mulligan was senior managing director at Sarissa Capital from 2013 to 2016, according to a statement Wednesday from Icahn, who said, "We are very pleased to have Richard join Icahn Capital given the depth and level of experience he brings as we look to enhance our focus on the biotechnology sector."

Icahn recently took a large stake in Bristol-Myers Squibb, according to The Wall Street Journal, which reported the activist believes the company may be ripe for a takeover. Icahn's thesis in biotech investing generally has been that smaller companies should be sold to larger ones, a strategy he put to work in sales of ImClone to Lilly, MedImmune to AstraZeneca, Genzyme to Sanofi and Amylin to Bristol-Myers.

Since Denner and Mulligan left, Icahn hasn't focused as much on the biotech world. Hiring Mulligan — a professor emeritus at Harvard Medical School and visiting scientist at the Koch Institute for Integrative Cancer Research at MIT, signals he may be back, at a time when biotech valuations are 25 percent off their 2015 highs.