During AIDS, the acquisition of mutations in the HIV-1 gp120 envelope glycoprotein leads to the switch from primary R5 (CCR5-using) to highly cytopathic X4 (CXCR4-using) HIV-1 variants. Based on the already known sequence homology between IgV genes and the gp120-coding region of the env gene, as well as on somatic hypermutation of multiple proto-oncogenes, the somatic hypermutation hypothesis for the mechanism of R5-X4 HIV-1 switching is proposed as follows. This switch takes place in the germinal center (GC) B cells due to the aberrant somatic hypermutation of the gp120-coding part of the HIV-1 env gene. Activation-induced cytidine deaminase (AID) is required for this process. Activation through IL4R and CD40 is required both for infection of GC B cells with HIV-1 and for induction of AID expression in the same cells. B cell infection with R5 HIV-1 variants is the limiting stage in the process of the viral phenotypic switch during the asymptomatic period of AIDS. Overall up-regulation of CXCR4 coreceptor on the GC B cells and the CD4(+) T cells surrounding the GC provides the predominant replication and acquisition of the newly formed X4 HIV-1 variants.