Members of the Myc and Jun/Fos gene families have been found to be expressed in late stages of cutaneous T-cell lymphoma (CTCL) and may be responsible for the transition from low-grade to high-grade tumors. The composition of these complexes is an important parameter, as the different homo- and heterodimeric jun and myc complexes can have gene transcription activating or suppressing activities. We determined the composition of the jun and myc DNA-binding complexes in three CTCL cell lines and malignant cells of seven Sézary patients by electrophoretic mobility shift assays (EMSAs) and "supershift" assays in which specific antibodies against the different members of the tested gene families were included in the binding reactions. Complexes containing JunD were found in three cell lines and two patients. The three cell lines and one patient contained also c-Myc/Max heterodimers. Because c-Myc/Max heterodimers are strong gene transcription activators and are necessary for cell-cycle progression, they may play a role in the progression of CTCL. JunD may also promote cell-cycle progression and influence the expression of cell death survival genes. Interleukin-7 (IL-7) and IL-15, which have been identified as growth factors for CTCL cells, stimulated the DNA binding of JunD and two novel c-Myc recognition site (E-box) binding proteins, but not the DNA binding of c-Myc/Max heterodimers.