Shares of Sage Therapeutics (NASDAQ: SAGE) were crashing 51.2% lower as of 11:27 a.m. ET on Monday. The huge sell-off came after the company and its partner, , announced following the market close on Friday that the U.S. Food and Drug Administration (FDA) didn't approve Zurzuvae (zuranolone) for major depressive disorder (MDD).

It wasn't all bad news for Sage, though. The FDA did approve Zurzuvae for treating postpartum depression (PPD). However, the company was betting on winning approval in the MDD indication as well.

The big problem for Sage -- and the main reason the biotech stock is plunging -- is that Zurzuvae will have a much smaller market size to target now. Millions of Americans suffer from MDD compared to hundreds of thousands of women who experience PPD.

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Source Fool.com