IL21 and systemic lupus erythematosus: To this end, we examined pATR expression levels in S (EdU+) and G2-M (pH3+) phase–derived cells, following ex vivo activation (IL-21/CpGb/sCD40L cocktail for survival and mild induction of proliferation) (32–38) and EdU exposure of peripheral total B cells from patients with SLE and HC (fig.