We describe for the first time a patient who has only one copy of the EVI1- and MDS1-gene, because of an inborn chromosomal defect. These two genes are known to be important for the stem cell functioning of the bone marrow, and thereby essential for the ongoing formation of blood cells. The decrease in the production of white and red blood cells in the reported patient is thus very likely caused by the absence of these genes on one chromosome. Notable, it has been well known that when leukemic patients – who have an abnormal increase of white blood cells – acquire an activated EVI1 gene develop an even more aggressive form of their leukaemia. (By Maartje Nielsen, http://jmg.bmj.com/content/49/9/598 )