The second patient was a 13-year-old boy who was admitted to the hospital because of recurrent infections and petechiae. Lymphadenopathy, hepatosplenomegaly, and the infiltration of the central nervous system were not observed. A diagnostic test revealed: WBC 1,800/microL, platelet count 21,000/microL, hemoglobin level 4.1 g/dL, and 90% leukemic cells in the bone marrow. The boy was diagnosed with BCP-ALL in August 2017. He presented a good treatment response to steroids, but was FMC-MRD-positive on day 15. Therefore, he was classified as high-risk group. According to the ALL IC-BFM 2009 protocol cytogenetic analysis and FISH were performed on a bone marrow sample. GTG band staining revealed an abnormal male karyotype with a marker chromosome probably derived from chromosome 21. The FISH study showed no rearrangements in BCR/ABL1, KMT2A, or ETV6/RUNX1. However, FISH revealed 8-9 signals of RUNX1 in 91% interphase cells (Fig. 2A). CytoScan HD array revealed amplification of arr[GRCh37] 21q11.2q21.1(15006457_22491098)x4, including NRIP1 and MIR125B2 genes and arr[GRCh37] 21q21.3q22.3(28054448_44101079)x4 containing 12 genes: TIAM1, OLIG2, RUNX1, DYRK1A, ERG, ETS2, TMPRSS2, TFF3, ADAMTS1, TFF1, TFF2, and TMPRSS3. The size of amplification was 16.5 Mb. In addition, the test also showed deletion of band 21q22.3 (PTTG1IP, S100B, CSTB, U2AF1), loss of heterozygosity within the long arm of chromosome 12, and a mosaic loss of heterozygosity within the long arm of chromosome 1 (Fig. 2B). The patient qualified for matched unrelated donor bone marrow transplantation.