KRTAP24-1 (keratin-associated protein 24-1) is a structural component of hair fibers that contributes to hair shaft rigidity and resistance through cross-linking with hair keratins. Located on chromosome 21, this high-sulfur KAP is specifically expressed in human scalp hair 1. The 254-amino acid protein exhibits high serine, proline, and tyrosine content with characteristic C-terminal tyrosine-containing tandem repeats, and is localized to the middle/upper hair cuticle 1. KRTAP24-1 functions as part of the interfilamentous matrix surrounding hair keratin intermediate filaments, contributing to fiber mechanical properties through extensive disulfide bond cross-linking. Genetic variation in KRTAP24-1 influences hair fiber phenotypes—analysis of caprine KRTAP24-1 variants identified four distinct sequences with nine SNPs, where genotype significantly associated with cashmere fiber diameter, with homozygous AA genotype producing the finest fibers 2. Ovine KRTAP24-1 similarly shows polymorphism affecting wool fiber properties 3. While the protein contains multiple putative post-translational modification sites (N-glycosylation, O-glycosylation, and phosphorylation) 3, clinical significance in human hair disorders remains underexplored. The gene's evolution shows conservation across mammals but lacks close homology to other KAP families 3, suggesting a specialized role in cuticular fiber structure.