We report a girl with a history of Ph(−) pre-B-ALL and three years of disease-free survival admitting to our hospital for regular end of treatment checkup with an increased white blood cell count which in follow-up studies and molecular detection of BCR-ABL (p210) fusions gene had been diagnosed as a Ph(+) typical CML. The upcoming question in this case scenario is whether developed CML is a secondary leukemia due to previous ALL chemotherapy or just a relapse case of primary leukemia.

About 2–5% of acute lymphoblastic leukemia (ALL) cases in pediatric patients are infants with an unfavorable prognosis because of high relapse probability. Early detection of the disease is, therefore, very important. Despite the fact that leukemia in twins occurs rarely, more attention has been paid to it in genetic studies. In the present study, through cytogenetic testing, a special case of concordant ALL in monozygotic twins was presented with different outcomes. In spite of an acceptable initial consequence to medical treatment in twins, in another brother (Twin B), early relapse was observed. In the cytogenetic study, both twins expressed t (4; 11) (q21; q23) while twin A expressed t (2; 7) (p10; q10). No cases have previously reported this mutation. Whether this translocation has a protective role for leukemia with mixed-lineage leukemia (MLL) gene rearrangement is still unclear. The difference in the translocation identified in the identical twins is also subject to further investigations.