To show the proof-of-principle of an anti-tumor therapy using GnRH-II antagonists, nude mice bearing xenografted human breast tumors were treated subcutaneously with the GnRH-II antagonist [Ac-D2Nal1, D-4Cpa2, D-3Pal3,6, Leu8, D-Ala10]GnRH-II. The gene discussed is GNRH2; the disease is breast neoplasm.