N-myc and L-myc proto-oncogenes are expressed in many developing embryonic tissues. In contrast to expression of the closely related c-myc gene, N-myc and L-myc expression is very restricted in adult tissues. We show that small amounts of the L-myc RNA can be detected in normal adult testicular tissue. A high level of N-myc expression from a single-copy N-myc gene was found in a malignant tumor of the testis, histopathologically defined as a seminoma. This tumor also showed a decrease of mRNA from the retinoblastoma gene (RB), which is ubiquitously expressed in all normal tissues. Strikingly similar results on elevated expression of the L-myc and N-myc genes and a lack of RB mRNA have been reported in small-cell lung cancer cell lines, and are confirmed in our study.