ESR1 and breast cancer: In the present study 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 that was characterized by ER and PR positive immunohistochemistry, and HER2 expression at 1+/2+ without gene amplification, as assessed at a central laboratory.