EP and EP-like sites are regulatory enhancer elements found in the promoters of several viral and mammalian genes which, in humans, include the MIF-1 binding site (MIE) of the c-Myc gene, the X box of MHC class II promoters and a binding site in the PCNA (proliferating cell nuclear antigen) promoter. The EP-like sites present in the X box of MHC class II promoters are distinctly nonpalindromic sequences that contain only a single EP-homologous half-site. The EP-like element is bound by a ubiquitous nuclear protein complex that consists of homo- and heterodimers involving the RFX1, RFX2, RFX3, RFX4 and RFX5 proteins. The RFX proteins represent an essential class II transcription factor family that share several conserved regions, including a centrally located DNA-binding domain (DBD) and a C-terminal D region that facilitates dimerization. RFX4 is a 735 amino acid nuclear protein that, via interactions with other RFX proteins, can bind DNA and is thought to activate the transcription of target genes. Four isoforms, each of which exhibit different tissue specificity, exist due to alternative splicing events.