SP1 and B-cell chronic lymphocytic leukemia: Sp1 binding has been shown to a critical regulator of IL21R expression in T cells [11]; however, in CLL cells, our luciferase assays with the promoter containing the Sp1 binding site failed to show increased activity with CpG stimulation, although it does appear that Sp1 binding may play a role in endogenous expression of IL21R.