In a nude mice xenograft model of breast cancer (n = 5 per group), injection of leptin (0.1 μg/g) dramatically increased tumor volume and mass, reduced survival, exacerbated pulmonary metastasis, and elevated IL-8 and Ki67 expression in the tumor tissue (All P < 0.05) compared with PBS injection. The gene discussed is CXCL8; the disease is neoplasm.