The name applied to the abnormality of the chromosome of the pair number 22 in the marrow cells of patients with chronic myelogenous leukemia. The abnormality results in shortened long arm of chromosome number 22. The observation was made at the University of Pennsylvania and named the Philadelphia chromosome. Since its discovery, the piece of chromosome lost has been shown to stick (translocate) to chromosome 9 in most cases. Indeed, some of chromosome 9 sticks (translocates) to chromosome 22. This is referred to as a balanced translocation, because virtually equal lengths of partial chromosome arms exchange position.