Synchronous presentation of ETV6::RUNX1 fusion positive concordant B-acute lymphoblastic leukaemia in identical twin toddlers.

Concordant leukaemia in identical twins is rare. The likelihood of concordance of leukaemia in twins is near 100% in infancy, around 10% from 1 to 6 years of age, and rare at a later age with variable latency. Reporting of new cases of concordant leukaemia in twins is encouraged to contribute to data pool of this infrequent but exceptional condition; especially when the theories with respect to evolution, natural history and molecular evidence explaining concordant leukaemia in identical twins are still evolving.We discuss identical pair of monochorionic twin toddlers who were detected to have pallor and blood investigations revealed pancytopenia. Further work up including bone marrow studies revealed synchronous diagnosis of B-acute lymphoblastic leukaemia (B-ALL) with ETV6::RUNX1 fusion. Synchronous presentation of concordant leukaemia in identical twins is extremely rare. Index twins are the only second set of twins and first one beyond infantile age with synchronous presentation of B-ALL.