CXCR4 and HIV-1 infection: However, the fact that CXCR4-using variants seem to develop successfully only once during HIV-1 infection [112,132], the scarcity of R5 virus variants with intermediate genotypes, and the fact that the newly emerged CXCR4-using variants differ from coexisting R5 variants by more than the minimally required number of amino acid mutations [133] altogether suggest that the virus evolves to the CXCR4-using phenotype through less-fit intermediate stages.