An electron-lucent region within the virion distinguishes HIV-1 from HIV-2 and simian immunodeficiency virus.

Ultrastructural comparisons of immature or budding particles of human immunodeficiency virus (HIV) types 1 and 2 and simian immunodeficiency virus of macaques (SIVmac) revealed no significant difference between these genetically distinct, but related, viruses. However, a region encompassing the core of mature HIV-1 virions was found to be more electron lucent than that observed in HIV-2 and SIVmac. This ultrastructural distinction cannot be attributed to HIV-1-specific vpu, HIV-2/SIV-specific vpx, or virion-associated vpr gene products.