An attempt is made to organize our current knowledge about genetically determined disorders of keratinized tissue, which primarily affect the epidermal structural proteins. Type I defects are those involving a change in a single amino acid and are analogous to sickle cell anemia. Type II defects are associated with abnormal retention of a normal structural protein intermediate. Type III defects are related to alterations in the normal post-translational cross-linking seen in keratinized tissues. Type IV defects are associated with altered proportions of fibrous proteins and are analogous to thalassemia. In type V defects, primary genetic disorders of other tissues profoundly affect keratinization in a secondary fashion. Examples from genetic disorders of the hair and epidermis are used to build this conceptual scheme.