To further examine the impact of stromal fibroblast-derived ZEB1 on mammary tumour progression, we crossed Fib-cKO mice with the MMTV-PyMT transgenic mice, a mouse model of breast cancer that mirrors the multistep progression of human breast cancers23, and generated female cohorts with the genotypes of MMTV-PyMT;ZEB1fl/fl (designed PyMT-Fib-WT mice), MMTV-PyMT;FSP1-Cre;ZEB1fl/wt (PyMT-Fib-Het mice) and MMTV-PyMT;FSP1-Cre;ZEB1fl/fl (PyMT-Fib-cKO mice) (Fig. 2a). Here, FBL is linked to breast cancer.