IRF5 and rheumatoid arthritis: Genetic investigations were carried out in an attempt to discover a connection between RA and PBC, revealing some common genes, such as histocompatibility complex, class II, DQ beta 1 (HLA-DQB1), membrane metallo-endopeptidase-like 1 (MMEL1), interferon regulatory factor 5 (IRF5), cytotoxic T-lymphocyte-associated protein 4 (CTLA4), signal transducer and activator of transcription 4 (STAT4), and CXC chemokine receptor 5 (CXCR5).