Mader is a novel delayed early response gene encoding a nuclear protein. Upregulation of the Mader 2.7 kb mRNA requires protein synthesis and can be induced in a variety of human cell lines by serum stimulation and in freshly isolated lymphocytes by mitogens. mRNA levels reach a maximum by 2 h and return to basal levels by 6 h. Mader is highly conserved as cross-hybridizing DNA sequences were observed in species as diverse as Rhesus and S. cerevisiae. The Mader protein of approximately 55 kD has two proline rich domains and contains 15 potential phosphorylation sites, a nuclear localization signal, and multiple S(T)PXX motifs that are characteristic of regulatory DNA binding proteins. Monoclonal antibodies produced against Mader confirm that it is localized to the nucleus. These features of Mader suggest that it may play a role in growth regulation. Although Mader mRNA can be detected in most cell lines, only occasional immunoreactive cells were detected in normal human tissues. In contrast, uniform strong nuclear staining was observed in all malignant melanomas examined. The fact that only one of six benign melanocytic nevi examined showed evidence of Mader expression suggests that over-expression of Mader protein may be associated with the malignant transformation of melanocytes.