Gilead Sciences CEO John Martin is stepping down after two decades in that role, the biotech company said Friday.

Martin, 64, will become executive chairman of Gilead GILD , while the company’s current COO John Milligan will become the new CEO. Gilead’s stock price dropped more than 5% following the announcement.

Gilead, No. 118 on the Fortune 500, had impressed investors in recent years with huge growth in its revenues and profits, driven by sales of its market-leading hepatitis C drugs; last quarter its earnings jumped 68% from the prior year. But the company’s share price has stumbled lately as investors have become increasingly antsy for Gilead to acquire another biotech firm, and lawmakers have targeted the company in an investigation into high drug prices. Gilead shares have fallen 20% in the last month alone.

Martin was a runner-up for Fortune’s Businessperson of the Year in 2014, when Gilead’s annual revenue doubled, and he was named Morningstar’s CEO of the Year in 2015 after presiding over a 100-fold increase in Gilead’s stock price during his tenure.

But he has since come under mounting investor pressure. Martin had abstained from dealmaking during the pharma industry’s M&A frenzy in the last few years, joking to investors on a recent earnings call that he was “open to suggestions” for companies that Gilead should buy. (See Fortune’s story, “Can Gilead Keep Growing Without a Big Deal?“)

Last week Martin received a letter from Massachusetts’ attorney general warning him that the Gilead’s pricing of its hepatitis C drugs Sovaldi and Harvoni—which can cost up to nearly $100,000 per treatment—may violate trade laws.