Demonstration of the genuine iso-12p character of the standard marker chromosome of testicular germ cell tumors and identification of further chromosome 12 aberrations by competitive in situ hybridization. The recently developed competitive in situ hybridization (CISH) strategy was applied to the analysis of chromosome 12 aberrations in testicular germ cell tumors (TGCTs). DNAs from two rodent-human somatic cell hybrids, containing either a normal chromosome 12 or the p arm of chromosome 12 as their unique human material, were used as probes. Our results demonstrate a genuine iso-12p character of the standard marker chromosome in TGCTs. Moreover, variant markers were identified representing translocation products that also involve chromosome 12. 