Although the cell line MCF7 is widely used in breast cancer research, its cytogenetic properties have not been thoroughly investigated so far. As conventional G-banding analysis cannot resolve the complex chromosome aberrations, we investigated MCF7 cells using molecular-cytogenetic methods, with particular attention to the DNA amplification site on chromosome 20q. With spectral karyotyping we found numerous unbalanced chromosome translocations, and with comparative genomic hybridization we detected many quantitative genomic imbalances. Furthermore, we analyzed the amplified region at 20q with the candidate tumour susceptibility gene STK15 in detail by fluorescence in situ hybridization, whole chromosome painting, immunohistochemistry, Western blot and expression analysis. In MCF7 interphase cells we found increased copy number of the STK15 gene associated with overexpression of STK15 mRNA. Accordingly, STK15 protein is overexpressed as compared to normal human fibroblasts in Western blot analysis. Overexpression of STK15 mRNA and protein is disproportionally stronger than that expected from the single additional copy of the STK15 gene. These data indicate that the highly increased level of STK15 protein in MCF7 cannot be explained by gene amplification alone. Apparently, secondary mechanisms of gene up-regulation are involved. This observation may be of general interest with regard to the activation of oncogenes in tumour cells.