ESR1 and breast cancer: In the study of YKL-40 expression and clinical outcomes, several independent studies with large breast cancer cohorts from different laboratories including ours demonstrate that YKL-40 expressed by breast cancer is associated with estrogen receptor (ER−), progesterone receptor (PR−), and human epidermal growth factor receptor 2 (Her2/neu) (Kim et al., 2007; Roslind et al., 2007b; Shao et al., 2011).