To this end, we knocked down RUNX3 expression in the fibroblasts, grafted them with DCIS breast cancer cells under the skin of highly immunodeficient NOG mice and demonstrated that knockdown of RUNX3 in the exp‐CAFs could suppress the growth of tumors formed with the exp‐CAFs and cancer cells. This evidence concerns the gene RUNX3 and ductal breast carcinoma in situ.