To interrogate the functional significance of STAT1 in limiting the tumorigenic potential of breast tumours with low (MT/Shc2F/2F) versus high (MT/ShcA+/+; MT/Shc313F/313F) STAT3 activity, we deleted STAT1 by CRISPR/Cas9 genomic editing (Fig. 4a and Supplementary Fig. 7a). Here, STAT3 is linked to breast neoplasm.