ESR1 and metastatic neoplasm: Notably, we found that proteins involved in triple‐negative breast cancer (TNBC), such as GSTP1 and EPCAM [20, 21], were enriched in ERα‐N metastatic tumours, while proteins typically upregulated in ERα‐P disease, such as ALCAM and KRT18 [22], were enriched in ERα‐P as well as the ERα‐C metastatic tumours (Fig. 3A,B).