Cytokeratin 10 is a heterotetramer of two type I and two type II keratins. Cytokeratin 10 is generally associated with keratin 1. It is seen in all suprabasal cell layers including stratum corneum. A number of alleles are known that mainly differ in the Gly-rich region (positions 490-560). Defects in cytokeratin 10 are a cause of epidermolytic hyperkeratosis (EHK), also known as bullous congenital ichthyosiform erythroderma (BCIE) or bullous erythroderma ichthyosiformis congenita of Brocq. EHK is an hereditary skin disorder characterized by blistering and a marked thickening of the stratum corneum. At birth, affected individuals usually present with redness, blisters and superficial erosions due to cytolysis. Within a few weeks, the erythroderma and blister formation diminish and hyperkeratoses develop. Transmission is autosomal dominant, but most cases are sporadic. Defects in cytokeratin 10 are also a cause of annular epidermolytic ichthyosis (AEI), also known as cyclic ichthyosis with epidermolytic hyperkeratosis. AEI resembles clinical and histologic features of both epidermolytic hyperkeratosis and ichthyosis bullosa of Siemens.