A 67-year-old woman was diagnosed with advanced breast cancer (mucinous carcinoma, histological grade I, estrogen receptor (ER) (−), progesterone receptor (PgR) (−), human epidermal growth factor receptor 2 (HER2) (1+), Ki-67 (20%), PD-L1 (IC1), cT4bN1M1 (PUL), and cStage IV). Here, PGR is linked to mucinous adenocarcinoma.