Based on limited published evidence, KRTAP25-1 is a high-sulfur keratin-associated protein that functions in hair shaft structure. In the hair cortex, KRTAP25-1 contributes to an interfilamentary matrix embedding keratin intermediate filaments, essential for forming rigid, resistant hair shafts through extensive disulfide bond cross-linking with cysteine residues of hair keratins 1. The protein localizes to the cytosol. Related KRTAP family members show hair follicle-specific expression 1, with orthologs associated with fiber diameter variation 2.