A jun related cDNA and its corresponding genomic fragment were cloned from human cells and sequenced. Polymerase chain reaction analysis showed that this gene is the human homologue of the mouse jun-D gene despite the fact that the degree of amino acid sequence conservation between the two is much poorer (77.3%) than that found between the homologues of c-jun and jun-B (95-98%). The product of this gene binds an AP-1 site and upon cotransfection stimulates the activity of a promoter that bears an AP-1 site. The level of activation is comparable to that of v-jun and the activity of both is further stimulated by v-fos. Deletion mutants of the gene that lack the best conserved region in the activating domain are poorly active. However, our data suggest that the activating domain is not confined exclusively to the conserved regions. Interestingly, at high concentrations human jun-D displays decreased activity which cannot be explained by a simple self squelching model.