Intriguingly, the ability of PR to drive JAK/STAT-dependent transcriptional responses requires the so-called CD domain of PR-B, which is located in the N-terminal B-upstream segment (BUS) region of full-length PR-B – and absent in PR-A – and is essential for proliferative signaling in breast cancer cells [119, 120]. Here, RB1 is linked to breast carcinoma.