Melatonin, as an endogenous hormone, is associated with the moderation of the circadian rhythm. Besides controlling the sleep-wake cycle, many physiological functions of melatonin have been identified, such as antioxidant, immune modulating, and anti-inflammatory. The current study was designed to estimate the effect of different doses of melatonin on gene expression of (p53) in serum and micronucleus (MN) in bone marrow stem cells as cytogenetic test. This investigation was conducted at a Research laboratory in the Department of Biology, College of Science, Wasit University and was designed to evaluate the rate of micronucleus (MN) formation as a cytogenetic test and the level of protein p53 as a serological test. Melatonin (5, 10&15) mg/kg was used for seven consecutive days to know their influence on the parameters before mentioned. Results showed a significantly increased in gene expression (protein) of p53 at P≤0.05 for all three doses of melatonin that were used. While Micronucleus production significant decrease at P≤0.05 for the same doses above of melatonin, when all results above were compared with the control group. 
