In the MCF-7 breast cancer cells of group 2, the effect of leptin over cell proliferation was positive, increasing up to 28.57%, except for the concentration of 100 ng/mL, in which at 72 h of stimulation, the cell population decreased up to 26.68% (10 ng/mL, 48 h) and was maintained until 72 h at the concentration of 1000 ng/mL, where there was a 3.45% increase with regard to the control. The gene discussed is LEP; the disease is breast cancer.