PGR and cancer: Over time, fewer ER−/PR+ breast cancers have been reported in Surveillance, Epidemiology, and End Results cancer registry data from 4.5% in the early 1990s to 1% in recent years [7], and recent studies of ER−/PR+ breast cancer analysis by repeat IHC using updated methods produced many reclassifications and a significant reduction in the number of ER−/PR+ cases [8–10].