On testing cutaneous T cell lymphoma cell lines and skin lesions, we found that the transcription factors STAT2, STAT3, STAT5, and STAT6 (STAT, signal transducer and activator of transcription) were present in the nuclei of these cells and that the binding to their specific DNA binding sites was stimulated by interleukin-7 and interleukin-15. DNA binding studies also revealed the presence of three additional DNA factors in cutaneous T cell lymphoma cells that bound to the same sequences and could also be stimulated by interleukin-7 and interleukin-15. One of these novel factors was also present in the adult T cell leukemia cell line Jurkat and malignant T cells from the blood of Sézary syndrome patients, but not in normal peripheral blood lymphocytes. It may therefore be a marker of T cell leukemia. It seems to interfere with the binding of STAT1 to the sis inducible element, suggesting that the DNA binding activity of STAT1 in cutaneous T cell lymphoma cells is disturbed.