To test whether lack of receptor internalization could explain the poor in vivo response of MLL‐5 to PVEK, we compared internalization of the G4723A antibody in two PDXs with high CD123 expression but diverse in vivo responses to PVEK (ALL‐2, T‐C 40 days, MCR; MLL‐5, T‐C 11 days, PD2; Figures 1 and 2; Table 1). Here, PAF1 is linked to acute lymphoblastic leukemia.