Among patients without acute graft-versus-host disease (aGVHD), refractory CMV reactivation increased significantly (P = 0.007) with decreasing presence of the donor NKG2C gene (46.8% for NKG2Cwt/wt, 62.1% for NKG2Cwt/del, and 81.8% for NKG2Cdel/del). The gene discussed is KLRC2; the disease is acute graft versus host disease.