Some aCN-LOH regions encoding known oncogenic mutations such as JAK2, MPL, c-KIT, FLT3, RUNX1 on chromosomes 9p24.1, 1p34, 4q21, 13q12 and 21q22, respectively, are frequently recurrent in AML and MDS, resulting in a “double hit” or “homozygous mutations”. This evidence concerns the gene RUNX1 and myelodysplastic syndrome.