PGR and breast cancer: Luminal A breast cancer was defined as positive for ER and negative for HER2 in the CALGGB9344 trial [16], positive for ER and/or PgR with a low Ki67 labeling index (cut-off of 13%) in the BCRG01 trial [17], and positive for ER with a low Ki67 labeling index (cut-off of 20%) in the PACS01 trial [18].