Here, we noticed that knockdown of MICAL1 in breast cancer cells significantly down‐regulated both CDK4 and cyclin D protein levels, whereas MICAL1 overexpression could reverse those effects on CDK4 and cyclin D. In addition, the numbers of breast cancer cell in S phase of the cell cycle were significantly lower after silencing of MICAL1 compared with control group, while MICAL1 overexpression improved them. The gene discussed is CDK4; the disease is breast carcinoma.