In order to prove that increased serum IGF-1 levels are a risk factor, studies have shown that systemic recombinant human IGF-1 (rhIGF-1) may stimulate the growth of tumors directly by stimulating mitosis in athymic mice (12), while the reduced circulating IGF-1 levels delayed the onset of chemical and genetic factors in induced mouse mammary tumors (13). Here, IGF1 is linked to breast cancer.