First, it renders cells resistant to only CCR5-tropic, not CXCR4-tropic, HIV-1 strains; second, a shift in HIV-1 tropism to CXCR4 usage would occur when CCR5 is disrupted53; and third, an increased susceptibility to some virus infections has been observed54–56. The gene discussed is CXCR4; the disease is viral infectious disease.