The well-established model breast cancer MCF-7 cell line of the minimally differentiated, little invasive, luminal type A (expressing estrogen and progesterone receptors) (2, 3, 4) was used for the terminal differentiation induction by the treatment with type I neuregulin 1 (also named heregulin (HRG)), a ligand of HER2-positive breast cancer (5). Here, PGR is linked to breast cancer.