RFX is a regulatory factor which binds to the X box of MHC class II genes and is essential for their expression. The DNA-binding domain of RFX is the central domain of the protein and binds ssDNA as either a monomer or homodimer [1]. It recognize X-boxes (DNA of the sequence 5'-GTNRCC(0-3N)RGYAAC-3', where N is any nucleotide, R is a purine and Y is a pyrimidine) using a highly conserved 76-residue DNA-binding domain (DBD) [2].

1: MHC class II regulatory factor RFX has a novel DNA-binding domain and a functionally independent dimerization domain. Reith W, Herrero-Sanchez C, Kobr M, Silacci P, Berte C, Barras E, Fey S, Mach B; Genes Dev 1990;4:1528-1540. PMID:2253877

2: Structure of the winged-helix protein hRFX1 reveals a new mode of DNA binding. Gajiwala KS, Chen H, Cornille F, Roques BP, Reith W, Mach B, Burley SK; Nature. 2000;403:916-921. PMID:10706293