Shares of Myriad Genetics fell a day after UnitedHealthcare said it would no longer cover multi-panel genetic tests for behavioral-health diagnoses.

Myriad Genetics stock was down 21%, to $17.30, in midday Friday trading. Shares of the diagnostic-test maker have fallen 34% over the past month.

Starting Jan. 1, the UnitedHealth Group unit plans to discontinue coverage and remove prior authorization requirements for multi-panel pharmacogenetic codes, it said.

The decision affects certain UnitedHealthcare commercial plans and all UnitedHealthcare Exchange plans, the insurer said.

Myriad Genetics wasn't immediately available for comment.