To study genomic imbalances potentially involved in disease development and/or progression of childhood MDS, array-based comparative genomic hybridization (aCGH) is a helpful tool. Copy number alterations (CNA) of subtle chromosomal regions containing potential candidate genes, e.g., TP53 or RUNX1 can be detected. However, characterizing small and/or heterogeneous tumor subpopulations by high-resolution aCGH within a majority of normal cells is a challenge in MDS and requires validation by independent methods like FISH or quantitative PCR. For the identification of tumor-relevant CNA, the analysis of DNA isolated from purified granulocytes or myeloid populations instead of DNA from whole bone marrow (BM) cells is helpful to overcome some of these limitations.