PCNA and breast cancer: PCNA was originally described as an autoantigen in systemic lupus erythematosus patients.27 PCNA is involved in the catalysis of DNA replication and also functions in chromatin remodeling, DNA repair chromatid‐cohesion, and cell cycle control.28 PCNA is preferentially expressed in proliferating cancer cells, and is suggested to be a prognostic marker and potential therapeutic target for various types of cancer including breast cancer.29 Small‐molecule and peptide drugs targeting PCNA are in development to inhibit cancer cell growth and sensitize cells for chemotherapy and radiotherapy.