In the present work, we have investigated the role of gal-7 in regulating mammary tumor development using gal-7-deficient mice (KOG7) and the mouse model of breast cancer FVB-Tg(MMTV-Erbb2)NK1Mul/J in which an activated form of Erbb2 is specifically expressed in mammary tissue. This evidence concerns the gene ERBB2 and breast cancer.