Complex retroviruses, including Human Immunodeficiency Virus Type 1 (HIV-1), are characterized by the ordered temporal expression of the various viral gene products in infected cells. This effect is mediated by a novel class of RNA-sequence-specific regulatory proteins typified by the Rev trans-activator of HIV-1. Evidence suggests that Rev regulates HIV-1 gene expression by intervening in the normal pathway of eukaryotic MRNA processing and transport.