The effects of CDK6 expression on TNF-α-induced rheumatoid arthritis synovial fibroblast (RASF) proliferation and synovial hyperplasia in collagen-induced arthritis (CIA) mice were investigated by modulating the transcriptional level with a CDK6 expression inhibitor (indole-3-carbinol), CDK6 small interfering RNA (siRNA), and Cdk6-deficient mice. Here, CDK6 is linked to rheumatoid arthritis.