Major attention is being paid in recent years to the genes harbored within the so called Down syndrome Critical Region of human chromosome 21. Among them, those genes with a possible brain function are becoming the focus of intense research due to the numerous neurobiological alterations and cognitive deficits that Down syndrome individuals have. MNB/DYRK1A is one of these genes. It encodes a protein kinase with unique genetic and biochemical properties, which have been evolutionarily conserved from insects to humans. MNB/DYRK1A is expressed in the developing brain where it seems to play a role in proliferation of neural progenitor cells, neurogenesis, and neuronal differentiation. Although at a lower level, MNB/DYRK1A is also expressed in the adult brain where, as judged by the phenotype of mutant and transgenic animals, it may be involved in learning and memory. Nevertheless, most of the molecular mechanisms underlying these functions remain to be unraveled. In this review we compile and discuss experimental evidences, which support the involvement of MNB/DYRK1A in several neuropathologies and cognitive deficits of Down syndrome.