The majority of rare diseases are hereditary. But despite significant progress in genome research, in most cases their exact cause remains unclear. The discovery of the underlying genetic defect is, however, a prerequisite for their definitive diagnosis and the development of innovative approaches to their treatment. Scientists have now succeeded in using a new process with which all of the genes in the human genome can be analysed simultaneously. The process was used for the first time on three children in a family who suffer from a rare form of mental retardation (Mabry Syndrome).