Cytochrome P4501 (CYP1) family enzymes are most active in hydroxylating a variety of environmental contaminants including Polyaromatic Hydrocarbons (PAH), planar polychlorinated biphenyls and arylamines. CYP1B which belongs to the cytochrome P450 superfamily of genes, is involved in the oxidation of endogenous and exogenous compounds, and could potentially be a useful biomarker in fish for exposure to arylhydrocarbon receptors (AhR) ligands. In this study, a new complementary DNA (cDNA) of the CYP1B subfamily encoding 1B1 was isolated from Nile tilapia (Oreochromis niloticus) liver after intracoelomic injection with benzo (a) pyrene (BaP). The full-length cDNA was 2107 base pair (bp) long and contained a 5' noncoding region of 29 bp, an open reading frame of 1527 bp coding for 508 amino acids and a stop codon, and a 3' noncoding region of 551 bp, respectively. The deduced amino acid sequence of Nile tilapia CYP1B1 shows similarities of 79.7, 70.3, 65.7, 65.4, 65.0, and 63.7% with Plaice CYP1B1, Japanese eel CYP1B1, zebra fish CYP1B1, common carp CYP1B1, common carp CYP1B2 and Channel catfish CYP1B1, respectively. The phylogenetic tree based on the amino acid sequences clearly shows tilapia CYP1B1 and Plaice CYP1B1 to be more closely related to each other than to the other CYP1B subfamilies. Furthermore, real-time PCR was used for measuring BaP induction of CYP1B1 mRNA in different organs of tilapia (O. niloticus), using β-actin gene as internal control, and the results revealed that there was a large increase in CYP1B1 mRNA in liver (22.8), intestine (2.0) and muscles (1.3).Keywords: Oreochromis niloticus, benzo (a) pyrene, CYP1B1 cDNA, sequence analysis, real-time PCR
