Cytochrome P450s(CYPs)are important xenobiotic metabolizing proteins.Their gene feature and protein function have been well understood in mammals,while their potent drug metabolic activity in freshwater fish aroused aquatic biologists’ attention.Many DNA sequences of CYP3As have been discovered in teleosts such as killifish,rainbow trout and minnow.However,little has been referred to important freshwater economic fish in China.This study focused on CYP3A’s cDNA sequence of crucian carp and predicted its protein structure and function preliminarily.In this study,degenerate primers for CYP3A of crucian carp were designed on the basis of conserved regions of known CYP3A sequences from five teleosts.Using crucian carp cDNA as a template,the partial fragments of crucian carp CYP3A cDNA were amplified.An antisense primer for 5′-RACE and a sense primer for 3′-RACE were designed to obtain the full-length CYP3A cDNA sequence by 5′ and 3′RACE.The full-length of CYP3A cDNA for crucian carp is 1 769 bp with open reading frame(ORF)of 1 545 bp encoding 514 amino acids.The calculated MW was of 58.624 ku and a theoretical pI of 6.30.The Cytochrome P450 Nomenclature Committee has named this cDNA CYP3A136.The deduced amino acid sequence of crucian carp’s CYP3A136 showed high similarity with those of rare gudgeon,grass carp and fathead minnow.It contained the conserved heme-binding motif of cytochrome P450 monooxygenases(FXXGXXXCXG).Furthermore,we examined the gene expression levels of CYP3A136 in tissues by reverse transcription-polymerase chain reaction(RTPCR)assay.The results demonstrated that the highest levels of CYP3A136 mRNA were seen in liver and intestine,followed by kidney and gill,and lower level were seen in other tissues.