U.S. stocks were mixed at the open on Thursday as the S&P 500 and Nasdaq Composite rebounded from a Wednesday afternoon selloff, while a sharp drop in the shares of UnitedHealth Group weighed on the Dow.

A sharp decline in shares of UnitedHealth Group Inc. was dragging down the Dow Jones Industrial Average on Thursday, putting it on track for historical underperformance versus the S&P 500. The blue-chip Dow index was tumbling more than 1%,

The Dow and S&P 500 might do something that’s never happened before, and it’s all UnitedHealth’s fault. The Dow is down 500 points, or 1.2%, while the S&P 500 is up 0.4%; the S&P has never risen on a day that the Dow has fallen more than 1%,