Focus of research are firstly the molecular biology and genetic defects of several pediatric tumors (neuroblastoma, medulloblastoma and rhabdomyosarcoma) and secondly the overall organization of gene expression of the human genome. Both research lines are supported by a strong bioinformatics and systems biology approach. The pediatric oncology research ranges from analysis of patient material for prognostic research, via fundamental research on molecular pathways and pathologic mechanisms, to validation of targets for novel drugs. In general, we search for genes mutated in tumors of pediatric cancer patients and subsequently try to reconstruct the molecular pathways in which these genes function. This is done by inducible expression of transgenes in cell lines, followed by mRNA profiling using microarrays or SAGE. Expression constructs include dominant negative genes and siRNA constructs. In parallel, large tumor series are analyzed by microarrays. Bioinformatic approaches and further wet-lab approaches are used to reconstruct the molecular pathways and their complex interactions. Pathways studied include the cell cycle, the N-myc and Delta-Notch pathways and pathways of many homeobox proteins like Meis1, Phox2B, Msx1 and Otx1. RNA interference is used to validate components of these pathways for suitatable drug targets, or to validate available and experimental drugs, both in cell lines and mice. The data from all high trouhput mRNA analyses and the bioinformatic tools developed to analyse them, are also used in the project to analyse the expression profile of the human genome. In this program we analyze the mechanisms and principles underlying our observation that the human genome is subdivided in domains of high and low gene expression.