2013 has been a blockbuster year for Gilead Sciences ( GILD) -- shares of the $96 billion biopharmaceutical company have charged more than 70% higher since the start of the year. But that upward momentum hasn't kept short sellers at bay; it's only made them shout their value argument louder. As I write, GILD carries a short interest ratio of 10.01, indicating that it would take more than two weeks of buying pressure for short sellers to exit their bets against this stock. >>4 Biotech Stocks Under $10 Making Big Moves Gilead's business centers around creating and marketing drugs that fight infectious diseases, with a focus on HIV and hepatitis B and C. Gilead's HIV treatments are making waves in the health care field -- the all-in-one pill Atripla dramatically simplifies the daily regimen for HIV patients, for instance. The firm also made an $11 billion bet on treating hepatitis C through the acquisition of drug maker Pharmasset. That diversification should be welcome for investors in this concentrated stock. While patent expirations are a concern for Gilead, the firm's keystone HIV patents don't start expiring until 2018, ample time for the firm to develop new formulations and to keep adding other diseases to its hit list. Financially, GILD is in solid shape in spite of its transformational acquisition efforts, with $3 billion in cash and investments largely offsetting a $7.3 billion debt load. There's no question that Gilead isn't exactly cheap right now, but it's trajectory isn't showing any signs of slowing either.