Keratins K6 and K16 are expressed in suprabasal interfollicular epidermis in wound healing and other pathological conditions associated with hyperproliferation, such as psoriasis and are induced when keratinocytes are cultured in vitro. However, these keratins are also constitutively expressed in normal suprabasal mucosal and palmoplantar keratinocytes. Mutations in keratins have been reported in the basal keratin pair K5 and K14 in epidermolysis bullosa simplex and in suprabasal epidermal keratins K1, K2 and K10 in epidermolytic ichthyoses. Two families with autosomal dominant disorder of focal non epidermolytic palmoplantar keratoderma, have oral mucosal and follicular lesions in addition to the palmoplantar hyperkeratosis. Previous studies have shown linkage in these families to the type I keratin gene cluster at 17q12-q21 and this report shows that the cDNA of affected members of both families have novel heterozygous mutations in the expressed keratin 16 gene. These mutations (R10C and N8S) lie in the helix initiation motif of the 1A domain. These mutations do not appear to cause epidermolysis on light or electron microscopy, which may reflect differences in function, assembly or interaction of the 'hyperproliferative' or 'mucoregenerative' keratins from other major types of keratins. The mutations reported here are the first to describe the molecular pathology of focal non epidermolytic palmoplantar keratoderma.