PD-1 expression was present in 50–100% of these childhood B-ALL xenografts (n = 11) (75% in PAX5-alt B-ALLs (3 out of 4 B-ALLs), 50% in ETV6::RUNX1+ B-ALLs (2 out of 4 B-ALLs) and 100% in hyperdiploid B-ALLs (3 out of 3 B-ALLs)) (Figure 2A, Figures S4 and S5 and Table S2) and approximately in 31% of diagnostic (11 of 36 B-ALLs) and in 43% of relapsed (9 of 21 B-ALLs) childhood B-ALL primary samples (Figure 2B and Figure S6 and Table S3). This evidence concerns the gene PDCD1 and precursor B-cell acute lymphoblastic leukemia.