Interferon regulatory factor 4 (IRF4) also known as MUM1 is a protein that in humans is encoded by the IRF4 gene. IRF4 functions as a key regulatory transcription factor in the development of human immune cells. The expression of IRF4 is essential for the differentiation of T lymphocytes and B lymphocytes as well as certain myeloid cells. Dysregulation of the IRF4 gene can result in IRF4 functioning either as an oncogene or a tumor-suppressor, depending on the context of the modification.