HDAC1 and rheumatoid arthritis: For example, due to the reduced expression of H3K4 methyltransferase Ash1L, Tregs in RA patients express low levels of Foxp3 while Ash1L can enhances TGF-β/Smad signaling promotes Treg differentiation, inhibits histone deacetylase 1 (HDAC1), and reduces histone deacetylation of Foxp3 (45).