Human hair keratins were among the first to be studied but it is only recently that sufficient information has been obtained to gain a basic biologic perspective of these proteins. Hair keratins are members of the intermediate filament family of proteins, yet are sufficiently divergent from epidermal keratins to warrant separate classification: type Ia and IIa ("hard"/hair keratins) and type Ib and IIb (epidermal and other "soft" keratins). As with hair keratins from other species, the human proteins may be distinguished from their epidermal counterparts by a relatively higher cysteine content, 7.6% versus 2.9%, respectively. This feature reflects utilization of disulfide bonding in producing a tougher, more durable structure in the tissues in which the hair keratins are distributed. Although prominent in hair, their distribution is not strictly limited to this tissue. A number of molecular characteristics have been elucidated from human hair keratin gene studies including amino acid sequence data for a type Ia hair keratin. Studies of various pedigrees has revealed a fairly wide latitude of variation in human hair keratin expression that is tolerated without associated obvious hair phenotypic change. Thus, a foundation of knowledge regarding these proteins has emerged and continues to evolve.