Second, while previous studies have shown that genetic ablation of the V3 loop typically abrogates Env function, in one instance removal of three amino acids from each arm of the V3 stem from an R5X4 virus resulted in a virus that lost the ability to use CCR5, but could still mediate infection of cells expressing CD4 and CXCR4 [26]. This evidence concerns the gene CCR5 and infection.