US STOCKS-S&P 500 reaches three-week high as health stocks rally
* Regeneron, Amgen lead health care sector
* Twitter surges 8 pct after investor raises stake
* Yum! Brands hit by China woes
* Indexes close up: Dow 0.73 pct, S&P 0.8 pct, Nasdaq 0.9 pct (Updates to close)
By Noel Randewich
Oct 7 (Reuters) - U.S. stocks ended stronger after a volatile session on Wednesday, led by a rebound in biotechnology companies that pushed the S&P 500 to its highest level in three weeks.
Materials shares also rose, helped by rising gold and silver prices.
Investors' focus is turning to earnings, and Yum! Brands was the S&P's biggest loser after reporting weak sales out of China, which accounts for more than half of its revenues.
Yum! Brands slumped 18.83 percent after the restaurant operator cut its full-year profit forecast, citing a slower-than-expected recovery in its key China market and the drag from a strong U.S. dollar. Continuación...