Interferon regulatory factor 4 also known as MUM1 is a protein that in humans is encoded by the IRF4 gene, located at 6p25-p23. In melanocytic cells the IRF4 gene may be regulated by MITF. IRF4 is a transcription factor that has been implicated in acute leukemia. This gene is strongly associated with pigmentation: sensitivity of skin to sun exposure, freckles, blue eyes, and brown hair color. A variant has been implicated in greying of hair. The World Health Organization (2016) provisionally defined large B-cell lymphoma with IRF4 rearrangement as a rare indolent large B-cell lymphoma of children and adolescents. This indolent lymphoma mimics, and must be distinguished from, pediatric-type follicular lymphoma. The hallmark of large B-cell lymphoma with IRF4 rearrangement is the overexpression of the IRF4 gene by the disease's malignant cells. Transcriptional activator. Binds to the interferon-stimulated response element (ISRE) of the MHC class I promoter. Binds the immunoglobulin lambda light chain enhancer, together with PU.1. Probably plays a role in ISRE-targeted signal transduction mechanisms specific to lymphoid cells. Involved in CD8+ dendritic cell differentiation by forming a complex with the BATF-JUNB heterodimer in immune cells, leading to recognition of AICE sequence (5'-TGAnTCA/GAAA-3'), an immune-specific regulatory element, followed by cooperative binding of BATF and IRF4 and activation of genes.