sentences:  This CD4-independence is associated with spontaneous mutation in the "env" gene.The presence of a co-receptor, CXCR4, is sufficient for this mutant strain to infect human cells.The strain with this phenotype was found to have seven mutations in the sequence coding for gp120 and it is proposed that these mutations induce conformational changes in gp120 that allow the virus to directly interact with the co-receptor.