Because of this opposite clinical significance and the predicted miRNA‐mRNA interactions, we used transfection of hsa‐miR‐195‐5p miRNA mimics to one ER‐ and two ER+ breast cancer cell lines and identified hsa‐miR‐195‐5p as a negative regulator of CHEK1 independently of ER expression. This evidence concerns the gene ESR1 and breast cancer.