SDR9C7 (short chain dehydrogenase/reductase family 9C member 7) is a NAD+-dependent oxidoreductase essential for epidermal barrier formation 1. Its primary function is catalyzing the dehydrogenation of linoleate 9,10-trans-epoxy-11E-13-alcohol esterified in omega-O-acylceramides to produce the corresponding 13-ketone, a reactive moiety critical for covalent binding of ceramides to proteins in the corneocyte lipid envelope 12. This enzymatic activity is necessary for forming the functional corneocyte lipid envelope, a crucial structural component of the epidermal permeability barrier 3. SDR9C7 also displays weak retinol dehydrogenase activity, converting all-trans-retinal to all-trans-retinol in the presence of NADH 4, suggesting a role in vitamin A metabolism during epidermal terminal differentiation. Loss-of-function mutations in SDR9C7 impair epidermal barrier function, as demonstrated through knockdown studies showing morphological abnormalities and increased dye penetration 5. Pathogenic SDR9C7 mutations cause autosomal recessive congenital ichthyosis (ARCI), a cornification disorder characterized by generalized skin scaling, dry scaly skin, and erythroderma 67. SDR9C7 mutations represent approximately 1% of ARCI cases and are associated with abnormal lamellar bodies on ultrastructural examination 7. The disease mechanism involves defective corneocyte lipid envelope formation due to loss of the critical 9,10-epoxy-11E-13-ketone acylceramide species.