According to Berkshire's most recent filing, which is as of June 30, it has bought approximately 5 million shares of UnitedHealth Group. It's a notable position for Berkshire, but it's not a huge one -- UnitedHealth accounts for just 0.5% of its total portfolio.

This investment signifies Buffett's re-entry into UNH after liquidating his entire position back in 2010, indicating that he perceives significant value at

UnitedHealth Group shares popped Friday after Berkshire Hathaway announced it purchased 5 million shares of the insurance giant.