Fetal membranes enriched for amniochorion revealed the presence of a transcript for PR that was not PR-B, not PR-A or PR-S leading to conclusion that the cytoplasmic PR isoform present was either PR-C or PR-M previously observed in human breast cancer cells and isolated from human aortic endothelial cells, respectively [9,13,28,29]. Here, RB1 is linked to breast cancer.