ESR1 and breast cancer: PIK3CA mutations are present in approximately 30–40% of all breast cancers (Arsenic et al., 2014; Buttitta et al., 2006; Campbell et al., 2004; Jensen et al., 2011; Koboldt et al., 2012; Li et al., 2006; Maruyama et al., 2007; Saal et al., 2005), with the highest frequency in estrogen receptor (ER)‐positive and human epidermal growth factor receptor (HER)2‐positive breast cancers (Koboldt et al., 2012; Saal et al., 2005).