Schematic overview and SV analysis. (A) Monozygotic twins developed BCP-ALL with a 6-month difference in latency. Sampling took place at birth (dried neonatal blood spots), diagnosis (bone marrow), and in remission (peripheral blood, 2 years 11 months [Tw1] and 2 years 5 months [Tw2] after diagnosis). WGS and dPCR analysis were applied to samples as illustrated to uncovered constitutional and somatic variants and quantify key mutations, respectively. Blue and red circles represent Tw1’s and Tw2’s leukemia, respectively. (B) Circos plot showing somatic SVs in leukemias. Only chromosomes involved in rearrangements are displayed (1, 11, 12, 17, and 21). Colored lines illustrate how breakpoints have fused. Genes disrupted by or in close proximity of the breakpoints are indicated. Blue lines represent the shared complex rearrangement t(11;12;21)(q23;p13;q22), involving 2 inversions on chromosome 11 and generating a ETV6-RUNX1 fusion. Redlines represent SVs unique to Tw2: 2 subclonal translocations t(1;12) and 1 t(11;12;17). (C) dPCR detection and quantification of shared complex rearrangement t(11;12;21)(q23;p13;q22) at birth, diagnosis, and in remission of both twins. TaqMan assay targeted chromosome 11;12 junction sequence. Clusters of dPCR chip wells positive for internal reference control RPPH1 (red), target region (blue), reference and target (green), and with no amplification (yellow). Complex rearrangement readily detected at diagnosis but beyond detection at birth and in remission. Detection limit: 1 in 1000 copies. Images acquired from QuantStudio 3D Analysis Suite Cloud Software, version 3.1.6-PRC-build2 with default parameters. Z, zygote.