The minibrain (mnb) gene of Drosophila melanogaster encodes a serine-threonine protein kinase with an essential role in postembryonic neurogenesis. A corresponding human gene with similar function to mnb could provide important insights into both normal brain development and the abnormal brain development and mental retardation observed in many congenital disorders. Trisomy 21 or Down syndrome (DS) is the most frequent human birth defect. It is associated with mental retardation and a broad spectrum of physical abnormalities. A region on human chromosome 21 has been designated the Down syndrome critical region (DSCR) and when present in three copies, this is responsible for many of the characteristic features of DS, including mental retardation. We have isolated a human homologue of mnb from the DSCR. MNB encodes a 6.1 kb transcript which is expressed in foetal brain, lung, kidney and liver. Using a human probe, two major transcripts (6.1 and 3.1 kb) were identified in mouse and expression was detected in situ in several regions of the mouse brain, including the olfactory bulb, the cerebellum, the cerebral cortex, the pyramidal cell layer of the hippocampus and several hypothalamic nuclei. This expression pattern corresponds to the regions of the brain that are abnormal in individuals with DS and suggests that overexpression of MNB could have detrimental consequences in DS patients.