 Shares of Avidity Biosciences Inc. RNA, 1.13 tumbled 23.7 in premarket trading on Tuesday after the company said the Food and Drug Administration placed a partial clinical hold on new enrollment in a Phase 12 clinical trial trial evaluating a new treatment for myotonic dystrophy type 1, a progressive neuromuscular disease. Avidity said the partial hold is the result of a serious adverse event reported in one patient. 