Leptin can significantly inhibit human HCC. This effect is mediated by inducing the proliferation and activation of natural killer cells and directly inhibiting tumor growth. The decreased NK expression of inhibitory CIS and the overexpression of anti-proliferative STAT2 and SOCS1 proteins in HCC lines may emphasize the anticancer effect of leptin. The gene discussed is SOCS1; the disease is in situ carcinoma.