Arguing against this possibility is that the vast majority of transmission occurs by one or a limited number of CCR5-utilizing virions [41], [42], HIV infection is extremely rare in homozygous CCR5Δ32 individuals [43], [44], [45], [46], [47], [48], [49], and transmission of X4 virus would be expected to result in sustained replication, which has not occurred. The gene discussed is CCR5; the disease is HIV infectious disease.