Of the 281 children who had received posttransplant MMR, VZV, or both vaccines and subsequently had antibodies tested (most frequently 1-3 months post vaccination), protective antibodies were present at varying levels (VZV, 107 of 149 [72%]; measles, 130 of 152 [86%]; mumps, 100 of 120 [83%]; rubella, 124 of 125 [99%]). The gene discussed is MRC1; the disease is measles.