RB1 and breast cancer: To demonstrate a role for phosphorylated PRs in breast cancer stem cell biology, we performed mammosphere assays using our well-defined T47D cell model systems expressing either empty vector (EV PR-null), unmodified WT PR-B, point mutant KR PR-B (K388R), or point mutant S294A PR-B missing the consensus MAPK phosphorylation site Ser residue (Fig. 8).