The gene encoding the human interleukin-2 (IL-2) receptor consists of 8 exons spanning more than 25 kilobases on chromosome 10. Exons 2 and 4 were derived from a gene duplication event and unexpectedly also are homologous to the recognition domain of human complement factor B. Alternative messenger RNA (mRNA) splicing may delete exon 4 sequences, resulting in a mRNA that does not encode a functional IL-2 receptor. Leukemic T cells infected with HTLV-I and normal activated T cells express IL-2 receptors with identical deduced protein sequences. Receptor gene transcription is initiated at two principal sites in normal activated T cells. Adult T cell leukemia cells infected with HTLV-I show activity at both of these sites, but also at a third transcription initiation site.