Consistent with the identification of DYRK family kinases as cell cycle regulators [70] and with the observation of microcephaly in humans, Drosophila and mice with MNB/DYRK1A haploinsufficiency [3,71,72,73,74] impaired neuronal proliferation have been reported in both loss and gain of function of DYRK1A as well as in DS individuals [75,76,77,78]. The gene discussed is DYRK1A; the disease is microcephaly.