In a mouse model of activated ErbB2-driven breast cancer, it was found that STAT3 does not have a role to play in tumour initiation, but it has a dramatic effect on metastatic progression with a 12-fold reduction in the number of metastatic lesions in the lungs of animals with STAT3-null/ErbB2 tumours when compared to STAT3-wild-type/ErbB2 tumours. This evidence concerns the gene STAT3 and breast cancer.