Finally, in vivo experiments found that overexpression of CCAT2 in vivo can increase the drug resistance to 5‐Fu in BC‐resistant transplanted tumour tissue, but inhibition of mTOR pathway can reverse the effect of CCAT2 on 5‐Fu resistance in BC‐resistant transplanted tumour tissue, which was consistent with our cell experiment results. The gene discussed is CCAT2; the disease is breast cancer.