ESR1 and cancer: The insulin-like growth factor 1 receptor (IGF-1R) is a receptor tyrosine kinase that plays a role in cancer development and progression.[1–3] In breast cancer, its expression is positively correlated with the presence of the estrogen receptor (ER).[4,5] Approximately 40 to 60% of ER-positive tumors express IGF-1R, while expression in ER-negative tumors is only 10 to 20%.[6] In general, IGF-1R correlates with good prognostic markers, such as ER positivity, older age, lower grade and human epidermal growth factor receptor 2 (HER2)-negativity.