We have identified a novel 7.7 Mb del(8)(q23.2q24.11) in a patient progressing to acute myeloid leukemia (AML) following a 12-year stable phase of chronic myelomonocytic leukemia (CMML). A surprisingly high JAK2+ allelic burden of 92% at the time of AML led us to delineate the molecular aberrations relevant for leukemogenesis. While a frameshift mutation in the TET2 gene was stably present throughout the course of disease the JAK2 mutation was acquired after initial diagnosis of CMML. At progression aCGH revealed del(8q)(q23.2q24.11) encompassing various cancer relevant genes of which RAD21 and CSMD3 are of particular interest.