Intellectual disability (ID) is a heterogeneous disorder, and hitherto, over 1000 genes have been described to be involved in the etiology of ID. Through a large international collaboration, we collected a cohort of 27 patients with ID and congenital anomalies, and a deletion or protein truncating variant in the ZFHX3 gene. The most consistent phenotype of ZFHX3 aberrations has been determined by ID, postnatal growth retardation, feeding difficulties and recognizable facial characteristics. Although ZFHX3 has been implicated in neuronal development, this has not been studied thus far. Therefore, in this project, we will determine the role of ZFHX3 during neuronal development more precisely and assess how loss of function can be linked to disease. Hence, identifying binding partners and (in)direct targets of ZFHX3 and their respective changes during normal neuronal differentiation, represents the starting point of the proposed project. Subsequently, the molecular and morphological effects of ZFHX3 overexpression and knockout will be studied in vitro. In addition, zebrafish zfhx3 knockout models will be generated to study changes in neuronal networks and behavioural characteristics. Overall, this study will elucidate the role of ZFHX3 in neuronal development and provide insights why loss of function is causing disease.