A deletion in the human CCR5 gene makes the individual resistant to HIV-1 infection by altering the syrface receptor for the virus. The mutant (D32) allele can be distinguished from the normal (1) allele by RFLP analysis with a restriction enzyme. The 1 allele produces a 332-bp fragment and a 403-bp fragment; the D32 allele produces a 332-bp fragment and a 371-bp fragment. Heterozygotes produce three bands.