This CGGGG in/del is part of a polymorphic repetitive DNA region, which includes either 3 or 4 CGGGG repeats; the insertion of an additional CGGGG unit (the 4 × CGGGG allele) is the risk allele associated with increased IRF5 transcription in peripheral blood mononuclear cells (PBMCs) and cultured epithelial cells derived from the salivary glands of SS patients, possibly through the addition of Sp1 binding site in the gene promoter of the 4R allele [53, 77]. Here, IRF5 is linked to synovial sarcoma.