ESR1 and breast cancer: Usually, the ER-positive breast cancer subtypes, such as luminal A and luminal B, demonstrate a good prognosis with optimal five-year survival rates ranging between 80% and 85%, while the ER-negative subtypes (HER-2-positive and basal-like) are more challenging in their treatment and are linked to poor prognosis with a 50-60% five-year survival rate (Table 1).