In this video after a big day in the biotech sector, Motley Fool health-care analyst David Williamson takes investors through today's big winner and loser.

Idenix (NASDAQ: IDIX) is the winner for the day, up 10% after posting solid results in a proof-of-concept trial for its hepatitis-C drug IDX21437. Idenix has also been in the news lately for its lawsuit against Gilead (GILD 0.12%) for patent infringement over Sofosbuvir, more commonly known as Sovaldi. The drug is set to do $5 billion in sales in its first year, making the legal battle worthwhile for Idenix, but David doesn't see this as something that should factor into the investing thesis just yet. He also worries about how far behind Idenix is in the hepatitis-C space, with the competition already having drugs showing excellent results in much later stages of development.

Meanwhile, MannKind (MNKD 1.22%) was the big loser of the day, falling by about 10% after the FDA announced a delay to its PFUFA, or approval decision date, for the company's inhaled insulin Afrezza. After a massive victory for the company last week, with the FDA advisory committee voting overwhelmingly in favor of approving the drug, the FDA's decision to postpone has made some concerned that it will go against the advisory committee's recommendation. David, however, remains unconcerned and thinks that the sell-off today may have been overblown. He still sees a high chance for FDA approval here and gives some ways that he would like to start seeing MannKind prepare for the drug's launch.