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 at high levels in skin and at lower levels in the developing mammary gland. In skin, expression starts shortly after birth and reaches a first maximum at 9 days. A second peak of expression is observed at 3.5 weeks, then levels decline and remain low in the adult. In the developing mammary gland, expression is detected exclusively at the onset of puberty. Belongs to the PMG family.