The fact that overexpression of AGR2 in benign rat mammary cells causes them to metastasise in syngeneic rats in vivo (Liu et al, 2005), but the human breast cancer cell line isolated from a metastatic plural effusion (Soule et al, 1973) fails to exhibit aggressive behaviour in vitro and to metastasise in vivo (Clarke et al, 1990) despite expressing AGR2, is at first sight surprising. This evidence concerns the gene AGR2 and breast cancer.