AFF1 and acute lymphoblastic leukemia: As chemo-resistance is also an important feature of MLL-r infant-ALL, we compared the responses of CRISPRMLL-AF4+ ALL blasts to prednisolone and l-asparaginase with ALL patient-derived xenograft (PDX) samples and the SEM and KOPN8 MLL-r cell lines and found that CRISPRMLL-AF4+ blasts showed similar levels of in vitro drug resistance to previous reports of treatment-resistant patient samples34–36 (Supplementary Fig. 4i and Supplementary Table 2).