In the retrospective study by Kuukasjärvi T. et al., which included 50 patients not receiving any adjuvant treatment, the discordance rate in HR status between primary BC samples and matched metachronous BC recurrences was 36%; in this study, all conversion events consisted in the loss of ERα and/or PgR expression, while no ERα− to ERα+ or PgR− to PgR+ conversions were detected [81]. Here, ESR1 is linked to breast cancer.