Pharmaceutical sector mainstay Gilead Sciences (GILD -10.15%) has had many good days on the stock exchange, but Monday wasn't one of them. On the back of dispiriting clinical trial results for one of its investigational medications, investors sent the company's share price down by more than 10%. And that was on a day when the bellwether S&P 500 index ended in positive territory, rising almost 0.3%.Trodelvy didn't meet its latest primary endpointThat trial was a late-stage one for Trodelvy, an approved cance ...