ERBB2 and breast cancer: We also showed that Ki67 rapidly and persistently dropped in cohort C of the NA-PHER2 trial, in which we applied the block of HER2, cdk4/6, and ER in women with HER2low breast cancer characterized by centrally confirmed ER and PgR positive immunohistochemistry, and HER2 expression at 1+/2+ without gene amplification.