Knowledge regarding thein vivoandin vitroproperties of human immunodeficiency virus replication is rapidly expanding. In particular, studies into the mechanism of action of three genes essential for virus growth, thegaggene encoding the p24 viral core protein, therevgene required for the export of human immunodeficiency virus envelope RNA from the nucleus to the cytoplasm of infected cells, and thetatgene responsible for transcriptional and translational viral activation, have led to the discovery of mutations capable of dominantly disrupting the function of each of these three genes. Culturing the plasma of human immunodeficiency virus-infected individuals for transmissible virus has emerged as a sensitive and potentially predictive assay for disease progression. Both primate and murine models have been refined by the isolation of molecular clones capable of inducing acquired immunodeficiency syndrome in rhesus monkeys and by the demonstration that human immunodeficiency virus-infected mice treated with zidovudine exhibit retardation of viral spread.