The Minibrain (Mnb) gene belongs to a new protein kinase family, which is evolutionarily conserved, and probably plays several roles during brain development and in adulthood. In Drosophila, mnb is involved in postembryonic neurogenesis and in learning/memory. In humans, MNB has been mapped within the Down syndrome critical region of chromosome 21 and is overexpressed in the Down syndrome embryonic brain. It has been widely proposed that MNB is involved in the neurobiological alterations associated with Down syndrome. Nevertheless, little is known about the functional role that MNB plays in vertebrate brain development. We have recently shown [Hämmerle et al. (2002) Dev. Biol., 246, 259-273] that in early vertebrate embryos, Mnb is transiently expressed in neural progenitor cells during the transition from proliferating to neurogenic divisions. Here we have studied in detail a second wave of Mnb expression, which takes place in the brain of intermediate and late vertebrate embryos. In these stages, MNB seems to be restricted to certain populations of neurons, as no consistent expression was detected in astroglial or oligodendroglial cells. Interestingly, MNB expression takes place at the time of dendritic tree differentiation and is initiated by a transient translocation from the cytoplasm to the nucleus. Afterwards, MNB protein is transported to the growing dendritic tree, where it colocalizes with Dynamin 1, a putative substrate of MNB kinases. We propose that MNB kinase is involved in the signalling mechanisms that regulate dendrite differentiation. This functional role helps to build a new hypothesis for the implication of MNB/DYRK1A in the developmental aetiology of Down syndrome neuropathologies.