Co‐culture of CLL cells on a CD40L‐expressing feeder layer can promote their proliferation to some extent, and some studies suggest addition of IL‐4 and IL‐21 (Schleiss et al., 2019) or CpG ODN (Purroy et al., 2015) can aid proliferation, but neither method allows indefinite culture or a true representation of in vivo conditions. Here, IL21 is linked to B-cell chronic lymphocytic leukemia.