Heterotetramer of two type I and two type II keratins. Abundantly expressed in the differentiating cortex of growing (anagen) hair. Expression is restricted to the keratinocytes of the hair cortex and is absent from inner root sheath and medulla. Expressed in malignant lymph node tissue in breast carcinoma tissue. 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.