Kang YJ, Lee HB, Kim YG, et al. - In order to resolve the existing controversy on the recognized value of Ki-67 in breast cancer, researchers focused on its prognostic value based on progesterone receptor (PgR) expression in patients suffering from estrogen receptor-positive, human epidermal growth factor receptor 2- negative early breast cancer. They analyzed data from patients (n = 1,848) who were treated surgically for nonmetastatic invasive breast cancer at a single institution between 2009 and 2012. In relation to high vs low Ki-67 expression, they noted significantly worse recurrence-free survival (RFS) and overall survival (OS) only when the PgR was low (< 20%) ( and 0.005, respectively, for RFS and OS). In the presence of a high PgR, they found no significant variation in RFS and OS according to Ki-67. Findings highlighted the worth of Ki-67 as a prognostic factor only under low PgR expression level in early breast cancer. Experts recommended considering PgR in the assessment of prognosis of breast cancer cases using Ki-67.