PRMTs are a family of proteins that either monomethylate or dimethylate the guanidino nitrogen atoms of arginine side chains. PRMT1 methylates Histone H4 on R3 and acts synergistically with p300/CBP to enhance transcriptional activation by nuclear receptors. PRMT1 also methylates a number of non-histone proteins such as the orphan nuclear receptor HNF4, components of the heterogeneous nuclear ribonucleoprotein (hnRNP) particle, interleukin enhancer-binding factor 3 (ILF3), and interferon alpha and beta receptors. This suggests roles for PRMT1 in transcriptional regulation, mRNA processing and signal transduction. PRMT1 is also a coactivator for p53, methylating histone H4 to facilitate p53-mediated transcription.