ESR1 and cancer: In relation to cancer, there is a great variety of PROTACs formulated against oncogenic proteins including transcription factors such as BET, STAT3, androgen, estrogen receptors; transmembrane receptors like, FLT-3, the epidermal growth factor receptor (EGFR); or intracellular signalling mediators, such as BRAF, KRAS, the fusion protein BCR-ABL, or the pro-survival protein MCL1, among others (Sun et al., 2019).