The co-existence of DCIS and MCA indicates that cancer cells of MCA may transform through mucinous metaplasia of epithelial cells of DCIS, accompanied by estrogen and progesterone receptors expression loss and that the in situ carcinoma form of MCA can derive from the mucinous metaplasia of epithelial ductal cells [5]. The gene discussed is PGR; the disease is ductal breast carcinoma in situ.