FAAP100 (Fanconi anemia-associated protein 100, also designated FANCX) is a core component of the FA core complex required for interstrand crosslink repair. It associates with FANCB and FANCL to form the catalytic BLP100 subcomplex, which catalyzes monoubiquitination of FANCD2 and FANCI—a critical step in activating the FA DNA repair pathway. FAAP100 regulates the stability and nuclear translocation of the FA core complex and its recruitment to chr17. Deficiency of FAAP100 causes severe Fanconi anemia 12. Homozygous loss-of-function variants identified in affected families result in cellular hypersensitivity to interstrand crosslink-inducing agents and defective FANCD2 monoubiquitination, with patient-derived cells exhibiting FA-consistent phenotypes. Faap100-deficient mice show embryonic lethality, microsomia, malformations, and gonadal atrophy 2. The developmental abnormalities associated with FAAP100 loss-of-function variants are among the most severe described across FA complementation groups, indicating that the FA pathway is essential for human development 1. Beyond its role in inherited FA, FAAP100 is frequently upregulated in multiple cancer types and promotes tumor progression in lung adenocarcinoma through enhanced proliferation and migration 3. Associations with copy number amplification and promoter hypomethylation suggest FAAP100 functions as a pan-cancer oncogene driver 3.