"Key Insights for Today's Stock Market Opening"

Nvidia shares are falling on "triple witching" day, Gilead Sciences is gaining on positive HIV drug trial results, Sarepta Therapeutics soars after expanded FDA approval for its gene therapy, Chipotle is in focus ahead of a 50-for-1 stock split, and CarMax shares rise despite missing quarterly forecasts. U.S. stock futures are down following Nvidia's decline.