Regulatory factor X (RFX) proteins are transcriptional activators that recognize X-boxes using a highly conserved 76-residue DNA-binding domain (DBD). DNA-binding defects in the protein RFX5 cause bare lymphocyte syndrome or major histocompatibility antigen class II deficiency. RFX1, -2 and -3 regulate expression of other medically important gene products (for example, interleukin-5 receptor a chain, IL-5Ra). Fusions of the ligand-binding domain of the oestrogen receptor with the DBD of RFX4 occur in some human breast tumours. The X-ray structure of two copies of the DBD of human RFX1 (hRFX1) binding cooperatively to a symmetrical X-box has been published recently.
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