TNFSF13B and plasma cell leukemia: BAFF and APRIL expression was reported in other B-cell malignancies, namely non-Hodgkin's lymphoma, plasma-cell leukemia and Waldenstrom's macroglobulinemia;[8], [31], [51] APRIL as a soluble factor, whereas BAFF was detected both as soluble and membrane form.[8] Here, we identified a new APRIL isoform, APRIL-δ, lacking the consensus motif for furin convertase-mediated cleavage,[32] which may explain the surface APRIL seen in B-ALL cells.