ERBB2 and cancer: In recent years, great advances have been made in mBC due to the development of innovative treatments for all cancer subtypes, including several anti-HER2 agents for HER2+ mBC, cyclin-dependent kinase (CDK) 4/6 inhibitors, phosphatidylinositol 3-kinase (PI3K) inhibitors, selective oestrogen degraders and other endocrine-based therapies for HR+ HER2− mBC, immune checkpoint inhibitors and antibody–drug conjugates for triple-negative mBC and polyadenosine diphosphate ribose polymerase (PARP) inhibitors for BRCA-associated mBC.