In the hair cortex, hair keratin intermediate filaments are embedded in an interfilamentous matrix, consisting of hair keratin-associated proteins (KRTAP), which are essential for the formation of a rigid and resistant hair shaft through their extensive disulfide bond cross-linking with abundant cysteine residues of hair keratins. The matrix proteins include the high-sulfur and high-glycine-tyrosine keratins. Interacts with hair keratins. Expressed in the hair follicles. Numerous size polymorphism are present in KRTAP4 gene family, which are mainly due to variations in the sequence encoding cysteine-rich repeat segments (PubMed:15955084). Belongs to the KRTAP type 4 family.