ESR1 and breast cancer: Delving deeper into understanding of the role of the endocrine system in the development of TNBC tumours created the possibility to re-evaluate SERMs for treatment of this subtype of breast cancer.8,9 For instance, it was shown that a specific isoform of ERα (e.g. ERα36) mediates the oestrogen signalling pathway participating in specific transcriptomic signatures of TNBC.10,11 4-Hydroxytamoxifen (II, Fig. 1) is an active metabolite of tamoxifen (I, Fig. 1)12,13 (SERM of the first generation) serving as an antagonist of ERα in breast tissue.