A unique cDNA clone, termed cDEX, coding for dexamethasone-inducible novel rat cytochrome P450 was isolated and characterized. The restriction enzyme map of the cDEX was slightly different from the reported two CYP3A1 sequences, and several nucleotide substitutions were found. Compared to the CYP3A1 sequence reported as P450pcn1 by F. Gonzalez et al. (J. Biol. Chem. 1985, 260, 7435-7441), there were 26 substitutions of nucleotides leading to 11 amino acid changes and a six-base (two amino acid)-long deletion in the open reading frame. Comparison with the sequence of pP450IGC2 reported by V. Ribeiro and M. C. Lechner (Arch. Biochem. Biophys., 1992, 293, 147-152) showed 18 nucleotide changes resulting in eight amino acid substitutions and the six-base-long deletion in the coding region. cDEX and these clones shared 97.8 and 98.4% similarity in deduced amino acid sequences, respectively, while only 86.3% similarity with P450pcn2 (CYP3A2). RNA blot hybridization analysis indicated that the mRNA corresponding to cDEX was mainly induced in rat liver by the administration of steroidal agents.