CSMD1 (CUB and Sushi Multiple Domains 1) is a large transmembrane protein that functions as a complement regulatory protein, specifically inhibiting the classical pathway C3 convertase by acting as a factor I cofactor 1. The protein is predominantly expressed in the central nervous system, particularly by astrocytes and at synapses in both human and mouse brain 12. CSMD1 plays critical roles in neurodevelopment by regulating complement-mediated synapse elimination during developmental pruning, with knockout mice showing increased complement C3 levels, aberrant retinal input segregation, and fewer retinogeniculate synapses 2. Biallelic variants in CSMD1 cause a neurodevelopmental disorder characterized by intellectual disability, microcephaly, and polymicrogyria 3. The gene is also essential for reproductive function, as CSMD1 knockout leads to infertility in both sexes, with increased complement deposition in gonads and impaired gonadal development 4. Additionally, CSMD1 functions in tissue repair, with expression in ECM-secreting fibroblasts during lung fibrosis resolution 5. CSMD1 has been implicated as a tumor suppressor and is associated with schizophrenia risk, though its specific mechanisms in these contexts require further investigation 6.