Luminal BC (LBC), which accounts for roughly 70% of BC cases [2], is further classified into two subtypes: LBC A: ER+/PR+, low proliferation, and generally HER2−; and LBC B: ER+/PR+, higher proliferation, with some HER2+ cases. The gene discussed is ESR1; the disease is CDH1-related diffuse gastric and lobular breast cancer syndrome.