ITSN1 (intersectin 1) is a multi-domain scaffold protein that plays crucial roles in clathrin-mediated endocytosis and neurodevelopment 1. The protein functions by organizing and stabilizing endocytic protein interaction networks during clathrin-mediated endocytosis, recruiting key proteins including AP2, epsin1, and dynamin2 to facilitate vesicle formation 1. ITSN1 is recruited during endocytic site stabilization and growth phases, and its knockdown impairs endocytic protein recruitment 1. Beyond endocytosis, ITSN1 is critical for brain development and neuronal function 2. Disease-wise, ITSN1 haploinsufficiency is associated with multiple neurological conditions. Loss-of-function variants cause autism spectrum disorders (90% of cases), intellectual disability (86%), and epilepsy (30%) 23. Notably, ITSN1 protein-truncating variants confer substantially increased Parkinson's disease risk (odds ratio 10.5) 4. The clinical significance is substantial, as ITSN1 represents a moderate-risk autism gene with less severe cognitive impairment compared to highly penetrant autism genes 3. Located on chromosome 21, ITSN1 overexpression in Down syndrome may contribute to early-onset Alzheimer's disease through endocytic dysfunction 5.