text: The Dow Jones fell 23.15 points to 18,529.42.
  The wider S&P 500 was 1.23 points lower at 2,182.64, but the tech-heavy Nasdaq index gained 6.22 points to 5,244.60.
  The announcement that US cancer drug firm Medivation would be purchased by pharmaceutical giant Pfizer sent its shares soaring 19.7%.
  Pfizer shares were down 0.4%.
  Crude prices fell 3.4% on signs of rising supplies. Brent crude fell to $49.16 a barrel, while US oil dropped to $47.43.
  Oil giant Chevron dropped 0.4%, and Marathon Oil sank 7%.
  Aside from the oil price fall, investors are cautious ahead of the annual meeting of central bankers later this week in Jackson Hole, Wyoming.
  "It's oil and then we've got Jackson Hole hanging over us, and there is always some surprise that comes out of Jackson Hole," said Jack Ablin, chief investment officer at BMO Private Bank, in Chicago.
  Federal Reserve chairwoman Janet Yellen will deliver a speech on Friday, which will be scrutinised for clues as to the timing of the next US interest rate rise.
  Computer chipmaker Intersil jumped 19.8% after a report that Japanese firm Renesas was in the final stages of negotiations to buy it for $2.99bn.
target: (Closed): US shares were mixed, as a rise in biotech stocks offset a fall in energy shares caused by a sharp drop in the oil price.