Stimulating BCR with soluble anti-IgM significantly improves CLL proliferation in the CD40L/IL4/IL21 system (with trimeric recombinant CD40L), but this does not reach the proliferation rates induced in normal B cells (~50% CLL-cell proliferation after 6 days vs. ~90% normal B-cell proliferation after 4 days) [44]. Here, IL21 is linked to B-cell chronic lymphocytic leukemia.