BOSTON ( TheStreet) -- Some of Wall Street's biggest investors, including hedge fund honcho Steven Cohen, are jockeying for profits in the raging hepatitis C drug-stock rally. Cohen of SAC Capital purchased just over 3.9 million shares of Gilead Sciences ( GILD) in the fourth quarter, according to a 13F filing for the period ended Dec. 31, 2011. Gilead, of course, is now leading the race to develop a new all-oral therapy for hepatitis C after acquiring Pharmasset for $11 billion. Cohen's Gilead grab is his second-largest existing position increase after Apple ( AAPL) in the fourth quarter. Gilead shares are up 40% since announcing the Pharmasset deal in November, and here's why: Capital Research Global Investors, one of the world's largest investment fund managers, bought 14.6 million additional shares of Gilead during the fourth quarter, according to its 13F filing. Other mutual fund giants like Wellington Management picked up another 12 million new shares while T. Rowe Price bought another 4 million shares. Hedge fund other than SAC Capital were also active Gilead buyers in the fourth quarter. Visium Asset Management acquired just over 4 million shares to begin what is essentially a new position for the firm. Baker Brothers also jumped on the Gilead bandwagon with a new 2.9 million-share position in the fourth quarter. Adage Capital added 1.1 million new Gilead shares, regulatory filings show. The hepatitis C drug stock prompting the most speculative chatter these days is Idenix Pharmaceuticals ( IDIX). Investors have bid the stock's value up over 50% since the beginning of the year in anticipation of a lucrative takeout along the lines of Pharmasset and Inhibitex, which was gobbled up by Bristol-Myers Squibb ( BMY) for $2.5 billion last month. Boston hedge fund Brookside Capital Management continues to add to its big Idenix position, acquiring another 1.5 million shares in the fourth quarter, the 13F filing shows. Brookside owned almost 11 million shares of Idenix at the end of 2011. Other notable Idenix pick-ups in the fourth quarter include Baupost Group (added 3.1 million shares), Capital World Investors (added 1.7 million shares) and DE Shaw & Co. (added 1.6 million shares), regulatory filings show.