ESR1 and breast ductal adenocarcinoma: [7] reported that compared with HER2‐negative cancers, HER2‐low cancers tended to have a higher pathological stage and grade, a higher incidence of ductal carcinoma, a higher positivity rate for estrogen receptor (ER) and progesterone receptor (PR), and a lower Ki‐67 (nuclear proliferation antigen) index.