Unusual five amino acid insert within subtype C HIV-1 envelope contributes to dual-tropism (X4R5).

During the course of HIV infection, some HIV-1 viruses switch from using the CCR5 (R5) coreceptor to using CXCR4 (X4). Here, we describe two subtype C isolates from a Zimbabwean patient that switched from using R5 to using both R5 and X4 with an accompanying addition of five amino acids to the V3 loop region of envelope. The insert appears to be derived from the human genome rather than a duplication within HIV-1.