Philadelphia chromosome. Coloured light micrograph of a karyotype (chromosome set) with two defective chromosomes (red). One (right) is from chromosome pair 22, and one (left) is from chromosome pair 9. This defect causes chronic myelogenous leukaemia (CML). The 46 chromosomes of a human karyotype can be arranged in 23 pairs of a maternal and paternal chromosome. The second chromosomes of the 22 and 9 pairs are normal (not highlighted). An exchange of material between the defective chromosomes caused this translocational defect. In a bone marrow stem cell, this defect on chromosome 22 leads to the increased white blood cell count in CML.