STAT1 and breast cancer: Surprisingly, STAT1 deletion in MT/ShcA+/+ (STAT3High) mammary tumours significantly impaired tumour onset in CD8+/+ mice (∼40% penetrance) but was dispensable for tumour initiation in CD8−/− animals, suggesting that STAT1 contributes to the establishment of immune suppression in STAT3High tumours (Fig. 4e,f).