Heterotetramer of two type I and two type II keratins. Synthesis occurs immediately above a small population of matrix cells at the base of the hair bulb and the trichocytes lining the dermal papilla and extends upward through the matrix and ends in the lower part of the cortex of the hair shaft. The disease is caused by variants affecting the gene represented in this entry. There are two types of hair/microfibrillar keratin, I (acidic) and II (neutral to basic). Belongs to the intermediate filament family.