PRF1 and acute lymphoblastic leukemia: In contrast, the presence of Prf1-/- NK cells led to a drastically lower number of primary and secondary resistant tumour cell clones (primary resistant: B-ALL+Prf1-/- NK cells n=96 vs B-ALL+WT NK cells n=245; secondary resistant: B-ALL+Prf1-/- NK cells n=15 vs B-ALL+WT NK cells n=56) (Figures 4F and Supplementary S8B).