ARHGEF1 and breast cancer: Although we observed no major difference in the growth of primary mammary tumours in Arhgef1-deficient mice compared with wild-type controls when crossed onto the MMTV-PyMT background (Extended Data Fig. 1d), we noted a reduction in the frequency of metastatic nodules in the lungs of Arhgef1-deficient mice, when mice with similarly sized primary mammary tumours were analysed (Fig. 1e).