HOXB2 and breast carcinoma: An illustration for a tumorigenic expansion of a minor population is the finding that in “luminal”-type Wap-Cre;EN mammary carcinomas [24] and low-grade WAP-T tumors (this study), the embryonic K8+K14+ cellular subset [53] is significantly expanded, while in the adult normal mammary gland K8+K14+ cells constitute a rare population that overlaps with CD61+ luminal progenitors [24].