Single T-cell factor engineered HS5 cell were not sufficient to induce CLL cell proliferation, but co-culture with HS5-CD40L-IL4 led to a reproducible mild CLL cell proliferation (median 7% on day 7), while the HS5-CD40L-IL4-IL21 cells led to a proliferation of ~44%. This evidence concerns the gene IL21 and B-cell chronic lymphocytic leukemia.