With the aid of the htgs and dbEST databases, a novel cytochrome P450 cDNA was found by homology searches, and the corresponding gene was identified on chromosome 19. Nested PCR was used to amplify a full-length sequence of 1515 bp. The predicted 504 amino acid sequence displays 38-49% identity with CYP2 family members and the protein was designated CYP2S1. mRNA dot blot analysis demonstrated high expression levels in trachea, lung, stomach, small intestine, and spleen. The expression pattern was confirmed by Northern blot, which also revealed a single transcript of approximately 2.4 kb. Western blot analysis, using an antiserum directed against the C-terminus of the enzyme, detected a protein in human lung with the same mobility as recombinant CYP2S1. Subcellular fractionation and immunostaining revealed that CYP2S1 was localized in the endoplasmic reticulum. We conclude that CYP2S1 represents a novel abundantly expressed human P450 with potential importance for extrahepatic xenobiotic metabolism.