KRT81 is a member of the keratin gene family. As a type II hair keratin, it is a basic protein which heterodimerizes with type I keratins to form hair and nails. The predicted 495-amino acid HB1 protein has an 88-amino acid C-terminal domain. The amino acid sequences of human and sheep HB1 are 90% identical. Northern blot analysis detected HB1 transcripts in human scalp but not unhaired breast epidermis.

HB1 is more closely related to HB3 and HB6 than to HB5. In situ hybridization studies demonstrated that type II keratin genes are sequentially activated in the hair follicle in the following order: HB5--(HB1, HB3)--HB6. Both HB1 and HB3 mRNA syntheses begin 10 to 15 cell layers above the apex of the dermal papilla and end abruptly in the middle of the cortex.