CYP1B1 and neoplasm: Mice lacking expression of CYP1B1 showed normal development and had no observable phenotype, demonstrating that CYP1B1 is not required for mouse development (20), and CYP1B1-null mice were protected against PAH-induced tumours in most tissues suggesting that CYP1B1 is required for metabolic activation and thus the carcinogenic potential of PAHs (20).