The same efficacy with fewer pills is the headline, but it's the understory that makes Gilead Sciences' (Nasdaq: GILD) latest clinical trial success important for investors.

The phase 3 trial compared Gilead's new HIV drug, elvitegravir, with Merck's (NYSE: MRK) Isentress. Both drugs were given with a background of other drugs including a protease inhibitor boosted with Abbott Labs' (NYSE: ABT) Norvir.

Numerically, elvitegravir beat Isentress with 59% of patients hitting the target levels of virus in the blood compared to 57.8% for patients taking Isentress. But the trial could only show that elvitegravir was non-inferior to Isentress -- essentially it works no better or no worse. That's an acceptable outcome since Isentress has to be taken twice a day, but elvitegravir is taken half as often.

Much more importantly, though, elvitegravir is part of a quad-pill together with boosting agent cobicistat, Emtriva, and Viread. The latter two are already on the market as individual pills, a combined pill called Truvada, and in Atripla, a triple combination that also includes Bristol-Myers Squibb's (NYSE: BMY) Sustiva. Gilead is also working on gaining FDA approval of a combination of the two with Johnson & Johnson's (NYSE: JNJ) TMC278.

With three of the four drugs now proven successful, that just leaves booster cobicistat, formerly GS 9350, as the wild card. And of course the combination has to work together to keep the virus at bay. Phase 2 data certainly gave that indication.

The quad-pill is important to Gilead because it owns all four components of the drug. Unlike Atripla or the combo with TMC278, the company won't have to share substantial revenue on the quad.

Investors won't have to wait too much longer before they know if the quad is approvable. Two phase 3 trials should finish up later this year, which could result in an early 2012 marketing application and an approval later that year.

Interested in keeping track of Gilead as it tries to get the quad approved? Click here to add it to My Watchlist, which will help you keep track of all our Foolish analysis on Gilead.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.