A prominent feature of retrovirus-induced immunodeficiency in mice (MAIDS) is early polyclonal activation of CD4+ T cells followed by the appearance of monoclonal lymphomas marked by clonal proviral integrations. These events appear to occur independent of interleukin-2 (IL-2), suggesting the activity of an alternative growth-promoting pathway. We studied the possible contributions to T cell expansion of a gene, Gfi-1, previously shown to confer IL-2 independence to rat T cell lymphomas.