CD33 and acute lymphoblastic leukemia: While previous studies suggested that one CD33 antibody (clone Him3-4) specifically recognizes the C2- set Ig-like domain and could be used to detect CD33 variants that lack exon 2 [9], in our experiments Him3- 4 only recognized CD33FL and CD33E7a but not CD33∆E2 or CD33∆E2,E7awhen expressed in virally-transduced ALL cell lines that are devoid of endogenous CD33 (Jurkat, RCH-ACV, REH, or RS4;11: Figure 3) and only recognized CD33FL but not CD33∆E2 when expressed in virally-transduced HEK293T cells (Figure 4).