Approximately 6% of paediatric patients with precursor B-cell acute lymphoblastic leukaemia (B-ALL) harbour a rearrangement involving the gene regions of PBX1 (1q23) and E2A (19p13.3) which is visualized cytogenetically either as a der(19)t(1;19)(q23;p13.3) or the less common balanced t(1;19)(q23;p13.3).