Gilead Sciences (NASDAQ:GILD) is up 6% today after posting solid earnings Tuesday. It's a well-deserved jump, but...

Let's save that "but" for the end -- first, the good news.

Sales of Gilead's HIV medications remain strong. Top-selling Atripla was up 9% year over year in the third quarter. Truvada was up 8% year over year.

And newcomers Complera and Stribild are building nicely. Complera, which Gilead developed with Johnson & Johnson (NYSE:JNJ), is almost at a $400 million run rate. Stribild just launched in the third quarter, so the $17.5 million was essentially just wholesaler stocking, but management said that prescriptions were trending almost twice as high as Complera was at the same point in its launch .

Stribild is Gilead's most important drug because the company owns all the components of the cocktail pill. For Complera and Atripla, Gilead has to share revenue with Johnson & Johnson and Bristol-Myers Squibb (NYSE: BMY), respectively.

Longer term, increasing sales of Stribild and Complera will help remove investors' concerns about the inevitable drop in revenue once its older medications lose patent protection. Atripla and Truvada combined made up 69% of Glead's revenue in the third quarter.

Sales of existing drugs are nice, but Gilead isn't up 68% this year because of hope for its existing drugs. Its next-generation hepatitis C pipeline will either justify its current valuation (and hopefully raise it) or cause shares to sink if the drugs don't live up to expectations.

With Gilead in the lead, those expectations are pretty high. Getting to the market first will help, but if Abbott Labs (NYSE:ABT), Bristol-Myers, Vertex Pharmaceuticals (NASDAQ:VRTX), Idenix Pharmaceuticals (NASDAQ:IDIX), or some combination thereof can get on the market only slightly behind and produce better cure rates and/or have a better side effect profile, Gilead's drugs aren't going to meet investor expectations.

Fool contributor Brian Orelli has no positions in the stocks mentioned above. The Motley Fool owns shares of Johnson & Johnson. Motley Fool newsletter services recommend Gilead Sciences, Johnson & Johnson, and Vertex Pharmaceuticals. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.