Overexpression of the Myc genes promote cellular transformation. Max protein exerts a pivotal function with the Myc family, and Mxi1/Mad proteins play a positive and negative activity, respectively, on transcription. The function of Mxi1 suggests that it might be a tumor suppressor protein. The Mxi1 gene map to 10p24-25 and deletions of this locus are frequently observed in prostate cancers. The N-terminal helical motif, helix-loop-helix (HLH) and leucine zipper (ZIP) regions of the Mxi1 gene are functionally important. We analyzed most of the coding region of the Mxi1 gene, including these three important regions in 32 prostate cancers and three cell lines by PCR-single strand conformational polymorphism (SSCP). To enrich neoplastic cells, the microdissection was performed on clinical samples. We detected a silent mutation in the HLH region, but no point mutations reflecting the functional change of Mxi1 were found in human prostate cancers. Point mutations of the Mxi1 gene, if they occur, must be minor events in primary prostate cancers.