IGF1 and neoplasm: However, treatment of estrogen-dependent cancer with anti-estrogens frequently evolves in drug resistance.17 Another factor controlling tumor Leydig cell proliferation is IGF-I, whose production is increased in rat Leydig tumor cells,16 so the use of drugs targeting IGF receptor (IGF1R), blocking IGF-I effects, could also be suggested for the treatment of this type of tumor.