As one astute Motley Fool Rule Breakers member pointed out, "The FDA is like a box of chocolates: You never know what sort of ruling you're gonna get." On Friday, biopharma giant Genentech (NYSE: DNA) received word that the FDA had approved its lead drug, Avastin, as a treatment for breast cancer.

Genentech may have been expecting approval on Avastin for breast cancer, but many analysts, like me, weren't. The FDA's decision is surprising because the agency sounded skeptical about the drug's survival benefits before an advisory-panel hearing earlier in the year. Then, in a split decision, the advisory panel recommended against approving Avastin for breast cancer because the benefits didn't appear greater than the risks.

Avastin is already approved as a breast cancer treatment in Europe. But the approval in the U.S. is not a totally done deal. For Avastin to continue as an approved drug for the treatment for breast cancer, the FDA will review the data from a recently completed phase 3 study of the drug and another phase 3 trial that will be completed in "late 2008," according to Genentech's estimates. If the cumulative results of these studies hurt Avastin's case as a breast cancer treatment, it could suffer the same fate that AstraZeneca's (NYSE: AZN) Iressa did after the FDA granted it early approval for lung cancer and then changed the label to state that no new patients should use the drug.

Avastin and its numerous label-expanding clinical trials will be the most important determinants of Genentech's future. In 2007, the drug accounted for 27% of Genentech's U.S. sales. Genentech said during its 2007 earnings announcement that whether it could reach the upper end of its guidance of $3.30 to $3.45 in adjusted earnings per share this year would depend on Avastin's getting this approval to treat breast cancer.

Investors shouldn't count all of their chickens before they're hatched, as the experience of Merck (NYSE: MRK), Schering-Plough (NYSE: SGP), and others with post-approval studies of their top drugs have shown. Friday's FDA decision is undoubtedly good news for Genentech, though, and its future remains bright, unless something goes very wrong with the other Avastin phase 3 trial due out later in the year.

Will this Avastin approval make Genentech a winner in 2008? Come and rate Genentech in The Motley Fool's free CAPS database, and see what other investors think about it.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A-plus disclosure policy.