ESR1 and papillary adenocarcinoma: Of note, tumors induced by RCAS-PyMT in TTA as well as TTB lines are papillary adenocarcinomas (Figure 2B), harboring a heterogeneous population of cells including keratin 8+ epithelial cells and keratin 5+ myoepithelial cells (Figure 2C), as well as cells stained positive for estrogen receptor α or keratin 6, a marker for biopotential mammary progenitor cells [26].