Hematopoietic stem/progenitor populations carry MLL/AF4. (A) Summary of MLL/AF4 positivity and 12 SNVs exclusive for the AML relapse, within different populations analyzed in patient LS01RAML. Circles with solid colors indicate VAF >30%, light dashed circle indicates VAF 5% to 30%. Remaining genes (open circle) represent the 10 other SNVs (of 12 SNVs) which showed the same pattern in the frequency of mutation acquisition (described in supplemental Table 8). (B) Summary of the proposed model of the origin of lineage switched relapse. Evaluation of B-cell receptor repertoires on ALL (presentation) and AML (relapse) lineage switched, and MPAL cases identified 3 different patterns. Pattern 1, with clonotypes on the ALL only. Pattern 2, B-cell receptor-containing clones on ALL and AML, but distinct to each other. Pattern 3, B-cell receptor-containing clones shared between ALL and AML. (C) BCR clone frequencies identified in whole-exome–seq data with application of MiXCR software in all lineage-switched acute leukemia (LSAL) and MPAL cases. VAF, variant allele frequency.