Keratin 15 is a type I cytokeratin. It is found in some progenitor basal cells within complex epithelia.Keratins can be divided into 2 subfamilies: acidic (type I) and basic (type II). A comparison of the amino acid and nucleotide sequences corresponding to different keratin polypeptides shows a constant principle of organization, namely, a conserved central rod of coiled-coil alpha-helical arrangement of 300 to 330 amino acids flanked by head and tail regions of highly variable sizes. Sequence homologies are restricted to the alpha-helical core portion. Members of the acidic subfamily display little, if any, amino acid homology to polypeptides of the basic one. Although in vitro any of the 10 to 15 acidic polypeptides is able to associate with any of the basic ones, the composition of the keratin pairs found in vivo is highly specific.