PGR and breast cancer: The incidence of breast cancer brain metastasis (BM) has increased in recent years, particularly in the subpopulation of triple-negative breast cancer (TNBC) (estrogen receptor <ER>-/progesterone receptor <PR>-/human epidermal growth factor receptor 2 <Her2>-, ER-/PR-/Her2-).1–3 TNBC, which accounts for 15–25% of all breast cancers,4,5 has problematic diagnosis because patients with TNBC do not respond to hormone or HER2-targeted therapies due to the lack of expression of ER, PR, and HER2.4 Moreover, the exact pathologic process of BM in the TNBC subset is poorly understood up to now.