SP1 and B-cell chronic lymphocytic leukemia: To assess the importance of Sp1 activity in regulation of IL21R in CLL cells, luciferase reporter constructs containing the Sp1 binding site of the IL21R promoter (the TGGGCG motif −49 to −44 bp from the major transcription initiation site [11], Figure 4a) were transiently transfected into CLL patient cells.