ESR1 and neoplasm: The lower amount of benefit seen in patients with high ER-expressing tumours is consistent with the observation in the recently published overview of trastuzumab trials in which ER+/PgR+ cases appear to receive less benefit than ER+/PgR− cases in which ER levels are lower and HER2 levels are higher (Arpino et al. 2005, EBCTCG 2021).