AsianFin -- UnitedHealth Group shares jumped more than 12% in premarket trading Friday after Warren Buffett’s Berkshire Hathaway revealed a new stake in the health insurer.

Berkshire owned 5.04 million UnitedHealth shares worth about $1.57 billion as of June 30, according to a U.S. Securities and Exchange Commission filing.

The investment comes as UnitedHealth’s stock has been under pressure from surging healthcare costs, a U.S. Department of Justice probe into its billing practices, a cyberattack, and the December shooting death of former top executive Brian Thompson. The shares have dropped nearly 46% this year, making it the worst performer on the Dow Jones Industrial Average.

Last month, UnitedHealth projected full-year adjusted earnings per share of at least $16, missing analysts’ already lowered expectations, while second-quarter results also fell short of Wall Street forecasts.

In premarket trading, the stock was last up 12.4% at $305.10.