In the UCBT group, 10 (40.0%) patients relapsed at a median time of 6.2 months (range 1.4–22.3 months) including one CD19-negative relapse and one central nervous system leukemia (CNSL), and one patient underwent transformation from ALL to myelodysplastic syndrome (MDS) and eventually progressed to acute myeloid leukemia (AML). Here, CD19 is linked to myelodysplastic syndrome.