We have carried out an analysis of amplified DNA sequences present in a tumorigenic mouse cell line, designated 3T3DM, to determine if the presence of cellular transforming activity is correlated with the elevated expression of any of the amplified genes. These studies utilized a selection protocol that allowed for DNA sequence amplification after the introduction of each gene into non-transformed recipient cells. Cell lines obtained from this selection protocol were assayed for tumorigenicity in nude mice. The results provided evidence that a gene, mdm2, that is amplified more than 50-fold in the 3T3DM cell line, induces tumorigenicity when experimentally overexpressed in NIH3T3 cells and in Rat2 cells. Analysis of the predicted amino acid composition of the mdm2 product(s) revealed features similar to those that have been shown to be functionally significant in certain DNA binding proteins/transcriptional activators. These include two potential metal binding motifs and a negatively charged domain rich in acidic amino acid residues. Overall, the data support the conclusion that mdm2 represents an evolutionarily conserved gene with tumorigenic potential and a predicted role in mechanisms of cellular growth control.