PROJECT SUMMARY While the cerebellum's role in motor function is well recognized, the cerebellum also plays cardinal roles in affective regulation, cognitive processing, and linguistic function (1). Indeed, there is a growing recognition that disruptions of cerebellar development cause considerable cognitive, behavioral, and social deficits (2-6). Yet, though cerebellar malformations are amongst the most commonly recognized structural brain malformation in prenatal imaging (7-10). Reliable information about their cause is sparse (11, 12). Diagnosis is based on imaging studies which are often unreliable, a problem amplified during fetal development (13, 14). In stark contrast to the wealth of knowledge gained over the decades regarding the mechanisms and genes driving cerebellar development in mice and other model organisms (15-19), we actually know very little about human cerebellar development. We recently reported multiple aspects of human cerebellar development significantly differing from mice and even rhesus macaque, a non-human primate. These discoveries challenge our current mouse-centric models of normal cerebellar development and the pathogenesis human cerebellar developmental disorders (20). This proposal seeks to advance knowledge of normal developing human cerebellum and cerebellar birth defects, leveraging 1) our unique access to normal and abnormal human fetal cerebellar tissue and 2) our extensive, specific expertise of mouse and human cerebellar development and our deep knowledge of human cerebellar malformations. Our detailed characterization of normal and abnormal cerebellar development, combined with humanized mouse models will improve our understanding of the biology of normal human cerebellar development and the pathogenesis of a clinically important human cerebellar birth defect, Dandy-Walker malformation (DWM). They will provide gold standard histological and transcriptomic datasets to assess model systems of human cerebellar development, generate the first “humanized” mouse models of human cerebellar development and finally, enable improved and sorely needed prenatal diagnostic information for families affected by cerebellar malformations.