الفهرس 
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Abstract
The cytochrome P450 superfamily (CYP) is a large and diverse group of enzymes that catalyze the oxidation of organic substances such as polycyclic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and aryl amines. Expression of cytochrome P4501A (CYP1A) has been measured as a biomarker for possible exposure to these pollutants. Using a quantitative reverse transcription-polymerase chain reaction (Q-RT-PCR), we estimated levels of CYP1A expression in Oreochromis aureus from five different locations of Lake Manzala: 1) El-Gamil, 2) El-Inaniya, 3) El-Sirw, 4) El-Bashtir and 5) El-Temsah during December 2011. The area of Al-Qanater was included as a reference site from Nile River. Higher levels of CYP1A gene expression and PAHs concentrations were observed in O. aureus collected from different locations of Lake Manzala than the reference site. The CYP1A gene was highly expressed in the liver of all fish populations followed by gills, testes, ovaries and muscles, respectively. Muscles expressed the least CYP1A gene among the tissues studied at Lake Manzala while the expression of CYP1A in muscles at the reference site was almost negative. At the location level, El-Gamil fish population exhibited the greatest CYP1A gene expression among the studied locations while CYP1A gene expression of El-Sirw fish population was higher in gills and muscles than El-Gamil fish population. Al-Qanater and El-Temsah fish exhibited the lower expression levels in all the organs studied. Higher levels of 11 compounds of PAHs were found in El-Gamil region, followed by El-Sirw, El-Bashtir, El-Inaniya, El-Temsah and Al-Qanater regions,