DNA-microarrays allow the analysis of almost the complete gene expression program of tumor samples and normal control samples in a single experiment. This allows the processing of a large number of samples in a reasonable short time. Tumor specific gene expression profiles can be used for molecular tumor classification and as a new diagnostic tool. In addition, the identification of tumor specific genes can help to understand the biology of tumor cells and identified genes can be used for the development of new therapeutic strategies. However, the huge amount of data generated by DNA-microarrays creates new challenges for data analysis. In addition, accuracy and reproducibility of the available techniques require complementary methods for verification of DNA-microarray data.