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. Strong expression in narrowly defined pattern restricted to the lower and middle cortical regions of the hair shaft in both developing and cycling hair. During hair follicle regression (catagen), expression levels decrease until expression is no longer detectable in follicles at resting stage (telogen). Expression in skin and hair follicle is regulated by HOXC13 and by GATA3. Belongs to the KRTAP type 21 family.