FCHSD1 (FCH and double SH3 domains 1) is a member of the F-BAR family that regulates actin cytoskeleton dynamics through distinct tissue-specific mechanisms. The protein contains an N-terminal FCH (F-BAR) domain and two C-terminal SH3 domains 1. In cochlear hair cells, FCHSD1 localizes primarily to the cuticular plate where it enhances actin polymerization by binding to sorting nexin 9 (SNX9) via its F-BAR domain, significantly promoting SNX9's WASP-Arp2/3-dependent F-actin polymerization activity 2. Unlike its homolog FCHSD2, FCHSD1 does not directly bind WASP proteins but functions through SNX9 interaction 2. Disease relevance includes a role in porto-sinusoidal vascular disorder (PSVD), where a heterozygous variant (R183W) causes mRNA and protein stabilization, leading to mTOR pathway overactivation 3. This variant demonstrates autosomal dominant inheritance and was validated in mouse models that recapitulated human PSVD phenotypes including splenomegaly and portal vein enlargement 3. Additionally, FCHSD1 shows altered expression in various cancers and psoriasis, suggesting broader roles in cellular regulation 45. The gene maps to chromosome 5.3 and belongs to the FCFBS superfamily characterized by FCH, FBH, and SH3 domains 1.