ESR1 and breast cancer: Meanwhile, for the same set of breast cancer patients approximately 20% of ER states determined by immunohistochemical (IHC) methods gave different results for different pathologists (Dubowitz, 1991; Arihiro et al., 2007), especially for weak ER-positive samples (Hammond et al., 2010; Sheffield et al., 2016), which may reduce the accuracy of pCR prediction.