Hereditary palmoplantar keratodermas (PPKs) are a highly heterogeneous group of skin diseases characterized by thickening of the skin of the palms and soles due to abnormal keratinization. PPKs have been classified clinically into diffuse, focal, and punctuate forms according to the pattern of hyperkeratosis on the palms and soles. Two different forms of the diffuse PPKs have been described, diffuse epidermolytic PPK (EPPK) and diffuse non-epidermolytic PPK (NEPPK).

EPPK is inherited through an autosomal dominant pattern. It is linked to regions of the genome containing a keratin gene cluster and caused by mutation in the keratin-9 gene (KRT9) on chromosome 17q12. A mild form of EPPK can be caused by mutation in the keratin-1 gene (KRT1) on chromosome 12q. Keratins are a large family of structural proteins and are the major components of the cytoskeleton of keratinocytes.