Seminomas and nonseminomas represent the invasive stages of testicular (TGCTs) of adolescents and adults. Although TGCTs are characterized by extra copies of the short arm of chromosome 12, the genetic basis for gain of 12p in the pathogenesis of this cancer is not yet understood. We have demonstrated that gain of 12p is related to invasive growth and that amplification of specific 12p sequences, i.e., 12p11.2-p12.1, correlates with reduced apoptosis in the tumors. Here we show that three known genes map within the newly determined shortest region of overlap of amplification (SROA): DAD-R, SOX5, and EKI1. Whereas EKI1 maps close to the telomeric region of the SROA, DAD-R is the first gene at the centromeric region within the 12p amplicon. Although all three genes are amplified to the same level within the SROA, expression of DAD-R is significantly up-regulated in seminomas with the restricted 12p amplification compared with seminomas without this amplicon. DAD-R is also highly expressed in nonseminomas of various histologies and derived cell lines, both lacking such amplification. This finding is of particular interest because seminomas with the restricted 12p amplification and nonseminomas are manifested clinically in the third decade of life and show a low degree of apoptosis. In contrast, seminomas lacking a restricted 12p amplification, showing significantly lower levels of DAD-R with pronounced apoptosis, manifest clinically in the fourth decade of life. A low level of DAD-R expression is also observed in normal testicular parenchyma and in parenchyma containing the precursor cells of this cancer, i.e., carcinoma in situ. Therefore, elevated DAD-R expression in seminomas and nonseminomas correlates with invasive growth and a reduced level of apoptosis associated with an earlier clinical presentation. These data implicate DAD-R as a candidate gene responsible in part for the pathological effects resulting from gain of 12p sequences in TGCTs. In addition, our results also imply differences in expression regulation of DAD-R between seminomas and nonseminomas.