ESR1 and breast cancer: It should, however, be noted that splice variants of ESR1 in breast cancer differ importantly from splice variants of the AR, as ESR1 splice variants are also expressed in healthy breast tissue (Poola and Speirs, 2001), and full‐length AR and splice variants are typically absent in CellSearch‐enriched fractions of HBDs (Onstenk et al., 2015a).