Therefore, we assumed that the knockdown of CCND1b downregulated the ratio of CCND1b/a, inhibited the CDK4/CyclinD1‐pRB‐E2F1 signalling pathway, prevented tumour cells from passing through the G1‐S transition, resulting in cycle arrest, thereby decreasing the chemoresistance of MCF‐7/ADM cells. The gene discussed is RB1; the disease is neoplasm.