H2BC21 and systemic lupus erythematosus: Autoantigens involved in systemic autoimmune diseases such as systemic lupus erythematosus (SLE) include dsDNA, small nuclear ribonucleoprotein (snRNP), cardiolipin, and histone proteins (i.e., H2B, H3, H4) (Doyle et al., 2014; Rosen and Casciola-Rosen, 2016).