Structural chromosome rearrangements of the 3q26 region are described in 5% of patients with myeloid malignancies. Most often these are AML, MDS, CML. The MECOM gene, which is designated as MDS1 and EV1 complex locus, is involved in chromosomal rearrangements in the 3q26 region. Rebuilding MECOM results in its overexpression, which is associated with a very unfavorable prognosis. AML is most often translocation t (3; 3) or inv inversion (3) and patients with this karyotype change form, according to the WHO classification, a separate, prognostically unfavorable subgroup of AML. Chromosome rearrangements involving MECOM occur in karyotype alone or together with chromosome 7 monosomy. The aim of our work is to present a summary of current scientific knowledge with MECOM gene rearrangements and to demonstrate its clinical significance using case reports.