CSMD3 (CUB and Sushi multiple domains 3) is a transmembrane protein that plays critical roles in neurodevelopment, particularly in dendrite development and synaptogenesis 1. The protein is predominantly expressed in the developing brain, with specific roles in cortical neuron development and cerebellar Purkinje cell morphology 23. CSMD3 functions as a putative complement control protein that regulates synapse formation and neuronal maturation 2. Loss-of-function studies in mice demonstrate that CSMD3 deficiency impairs synaptogenesis, reduces neurogenesis, and causes abnormal cerebellar Purkinje cell dendritogenesis and spinogenesis 23. Clinically, CSMD3 variants are associated with multiple neurological conditions including focal epilepsy, febrile seizures, autism spectrum disorder, and other neurodevelopmental disorders 432. The gene spans over 1.2Mb with 73 exons and shows homology to other CSMD family members 1. CSMD3 dysfunction leads to altered synaptic plasticity, increased neuronal excitability, and behavioral phenotypes including social interaction deficits and motor dysfunction, suggesting its essential role in proper brain development and function 32.