We first investigated whether sericite could suppress the proliferation of human breast cancer cell lines BT474 (triple positive), MCF7 (ER and PR positive and HER2 negative), SKBR3 (ER and PR negative and HER2 positive), BT549 and MDA-MB231 (triple-negative), and MCF10A (normal breast epithelial cells). This evidence concerns the gene PGR and breast cancer.