Acute lymphoblastic leukemia (ALL) in monozygotic/monochorionic twins.
 Diagram illustrating sequence and developmental timing of critical mutational events in ALL in monozygotic twins (with one placenta) with either concordant (shared) ALL or discordant ALL (ie, in one twin only).
 Fusion gene (= initiating lesion).
 Copy number variation (secondary, postnatal gene deletions or gains).
 *Concordant/discordant percentages. Probability that second twin will develop ALL after it is diagnosed in first twin.
 f1, f2 Fetus.
 Transmission of pre-leukemic clone from one twin to the other (via intra-placental vascular anastomoses). Direction of transfer unknowable.