ESR1 and breast cancer: Whole genome sequencing (WGS) of spontaneous estrogen receptor alpha positive (ERα + ) Stat1−/− mammary tumors identified an mPRLr mutation hotspot (Supplementary Fig. 1a), leading to recurrent C-terminally truncated mPRLr mutants (mPRLrT; identified in 100% of sequenced primary tumors, and 0% of adjacent normal samples)13.