The biotechnology sector is having its worst day since April as investors fear health insurers and companies that manage patient's drug benefits will put new pressure on how much the industry can charge for breakthrough treatments. The selloff, prompted by Express Scripts Holding's announcement Monday that it would block its U.S. patients from getting Gilead Sciences's $1,100-a-pill hepatitis C medicine, sent the Nasdaq Biotechnology Index down 4.6 percent, the biggest one-day drop since April.