Our study showed that NGR1 was able to inhibit breast cancer cell proliferation by down-regulating RUNX2 protein and thus, interestingly, we found that MDA-MB-231 was more strongly inhibited by NGR1 compared to SK-BR-3, which may be related to the expression level of RUNX2 in breast cancer cells, which was low in SK-BR-3 cells compared to other breast cancer cells. The gene discussed is RTN4R; the disease is breast cancer.