A novel cytochrome P450 (P450 or CYP) isoform belonging to the CYP4 family was cloned with reverse transcription-polymerase chain reaction from rat brain. The nucleotide sequence contained an open reading frame coding for 507 amino acids. The deduced amino acid sequence showed 41-51% identity with that of members of the rat CYP4 subfamilies 4A, 4B, and 4F. The enzyme was designated CYP4X1. Northern blot analysis showed that CYP4X1 is highly and specifically expressed in the brain. In situ hybridization experiments suggest that CYP4X1 is mainly expressed in neurons in different regions, e.g., the brain stem, hippocampus, cortex, and cerebellum as well as in vascular endothelial cells. The function of this novel P450 enzyme is unknown, but the expression pattern of CYP4X1 suggests that it is possible that CYP4X1 plays a role in neurovascular function. The catalytic properties and physiological function of CYP4X1 are currently under investigation.