On the other hand, IKKα promotes the estrogen-induced transcription of E2F Transcription Factor 1 (E2F1) and facilitates the subsequent activation of several E2F1-responsive genes such as thymidine kinase 1 (TK1), proliferating cell nuclear antigen (PCNA), cyclin E, and cdc25A, which are required for cell cycle progression of breast cancer cells [22]. Here, PCNA is linked to breast cancer.