Some good news came out of Gilead (GILD 0.64%) this morning, as the company's drug Sofosbuvir was deemed safe enough to continue with a critical phase 3 trial. This drug is a vital part of Gilead's focus on hepatitis C, but this class of drugs has had some safety questions in the past. In the following video, Fool health-care analyst David Williamson tells investors what the key things to watch will be, now that Gilead has gotten the go-ahead to move into this major phase 3 trial.