ESR1 and breast cancer: This class of molecules include fulvestrant, a selective estrogen receptor downregulator (SERD) against estrogen receptor-α (ERα) (Osborne et al., 2004) which is approved in the clinic against breast cancers expressing ERα, and a selective androgen receptor downregulator (SARD) against androgen receptor (AR) (Omlin et al., 2015) currently under clinical evaluation.