US STOCKS-Wall St rises as health care rebounds
* Regeneron, Amgen lead health care sector
* Twitter surges 7 pct after investor raises stake
* Yum Brands hit by China woes, drags McDonald's
* Indexes up: Dow 0.29 pct, S&P 0.37 pct, Nasdaq 0.41 pct (Updates to afternoon)
By Noel Randewich
Oct 7 (Reuters) - U.S. stocks rose in a volatile session on Wednesday, led by a rebound in biotechnology stocks, putting the S&P 500 on track for its sixth day of gains out of the last seven.
Materials shares were higher, helped by rising gold and silver prices.
Investors' focus is slowly 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! slumped 19 percent, while McDonald's, which also has a large exposure to China, fell 0.5 percent to $101.12. Continuación...