Plays a central role in hair formation. Essential component of keratin intermediate filaments in the inner root sheath (IRS) of the hair follicle. Heterodimer of a type I and a type II keratin. Associates with KRT16 and/or KRT17 (By similarity). A complex variation, consisting of an 81-bp deletion, including the last 4 bp of intron 6 and the first 77 bp of exon 7, followed by 2 insertions was identified. This variation was called Rex polymorphism or 'Re'. All sequenced Devon Rex cats, a breed characterized by curly hair phenotype, were homozygous for this variation and loss of homozygosity was associated with normal hair production (PubMed:20953787). A variation affecting the 5'-splice site at the junction of exon 4 and intron 4 was identified in 2 breeds, Sphynx and Kohana, that are characterized by a hairless phenotype. This variation occurred in the heterozygous or homozygous state in these breeds. It has been called hairless or hr allele and has been suggested of a cause of the hairless phenotype (PubMed:20953787). Belongs to the intermediate filament family.