These tumors differ from those arising in mice harboring other Stat1-targeting KO constructs or in other mice that are knocked out for tumor suppressor genes such as Trp53 or Brca1. 129:Stat1−/− tumors share a signature histological and cytological phenotype, with ER+, PR+, and FoxA1+ LOP cells. This evidence concerns the gene ESR1 and neoplasm.