KRTAP13-3 (keratin-associated protein 13-3) is a structural protein component of hair fibers located on chromosome 21. Primary function: KRTAP13-3 belongs to the high-glycine-tyrosine (HGT) class of keratin-associated proteins that form the interfilamentous matrix within hair cortex, embedding hair keratin intermediate filaments 1. Mechanism: These proteins are essential for hair shaft rigidity and resistance through extensive disulfide bond cross-linking with cysteine-rich hair keratins, creating a mechanically stable fiber structure. Disease relevance: KRTAP13-3 has been identified as a frequently mutated gene in atypical parathyroid tumors, appearing among the most commonly altered genes in whole-exome sequencing studies of these neoplasms 2. Clinical significance: KRTAP13-3 DNA methylation at specific CpG sites serves as a biomarker for cellular senescence in therapeutic cell products, enabling quality control assessment of mesenchymal stromal cells through epigenetic analysis during culture expansion 3. The gene's methylation status correlates with replicative senescence and can predict cumulative population doublings in cell preparations intended for clinical use.