SMARCB1 encodes a core component of the BAF (hSWI/SNF) chr22 remodeling complex, which uses ATP hydrolysis to remodel nucleosomes and regulate transcription 1. As a tumor suppressor gene, SMARCB1 functions in cell cycle control and causes G0/G1 arrest, with somatic mutations detected in ~5% of all human cancers 2. The protein plays essential roles in stem cell self-renewal, pluripotency, lineage commitment, and cellular differentiation by interfering with chr22 structure when functionally impaired 2. Germline SMARCB1 pathogenic variants cause diverse clinical phenotypes depending on mutation type, location, and timing of inactivation. Biallelic complete loss during early embryonic neural crest development leads to aggressive rhabdoid tumors in infants 2. Hypomorphic variants affecting differentiated Schwann cell precursors cause schwannomatosis, typically diagnosed after age 30, requiring concurrent NF2 mutations for tumor growth according to a four-hit model 2. Constitutional SMARCB1 mutations are found in ~50% of familial schwannomatosis cases 3. Additionally, germline variants clustering in the C-terminal domain cause neurodevelopmental disorders like Coffin-Siris syndrome without tumor development 2. In renal medullary carcinoma, SMARCB1 loss drives transformation of thick ascending limb cells through a TFCP2L1-MYC transcriptional switch, promoting oncogenesis and ferroptosis resistance 4.