To test whether CCR5 gene-disrupted cells had a selective advantage over CCR5 expressing cells during R5-tropic HIV-1 infection, we mixed CCR5-/CR2 and mock-transduced CEMss-CCR5 cells at 1∶9 ratio and then challenged cell mixture with either X4-tropic HIV-1 Bru-3 or R5-tropic HIV-1 Bru-Yu2. The gene discussed is CCR5; the disease is HIV-1 infection.