Objective: To identify the causative gene in a large Dutch family with familial cortical myoclonic tremor and epilepsy (FCMTE). Methods: We performed exome sequencing for 3 patients of our FCMTE family. Next, we performed knock-down (shRNA) and rescue experiments by overexpressing wild-type and mutant human d-catenin (CTNND2) proteins in cortical mouse neurons and compared the results with morphologic abnormalities in the postmortem FCMTE brain. Results: We identified a missense mutation, p.Glu1044Lys, in the CTNND2 gene that cosegregated with the FCMTE phenotype. The knock-down of Ctnnd2 in cultured cortical mouse neurons revealed increased neurite outgrowth that was rescued by overexpression of wild-type, but not mutant, CTNND2 and was reminiscent of the morphologic abnormalities observed in cerebellar Purkinje cells from patients with FCMTE. Conclusions: We propose CTNND2 as the causal gene in FCMTE3. Functional testing of the mutant protein revealed abnormal neuronal sprouting, consistent with the abnormal cerebellar Purkinje cell morphology in patients with FCMTE.