evidence for aberrant activation of the interleukin2 autocrine loop by htlv1encoded p40x and t3ti complex triggering in this study we provide evidence that distinct dna sequences within the 5flanking regions of the genes for interleukin2 il2 and its receptor il2r are involved in human tcellspecific activation of transcription by p40x a product of human t cell leukemia virus type i htlv1 the same dna sequences appear to be responsible for induction of the genes in a t cell line jurkat by mitogens although the il2 gene sequences are activated by p40x with much lower efficiency than the il2r gene sequences they are synergistically activated by the p40x expression and subsequent extracellular stimulation by concanavalina or antit3 we propose a model for twostep activation of the il2 autocrine loop in atl development