AFF1 and acute lymphoblastic leukemia: As also observed by others [4], while two MLL-AF4 ALL cell lines (SEM and RS4;11) displayed high levels of CD133 expression, CD133 was undetectable on the surface of THP1 (MLL-AF9 AML), MV4;11 (MLL-AF4 AML), and RCH-ACV (non-MLLr ALL) cells (Fig. 1a).