Philadelphia chromosome. Coloured light micrograph of two chromosome pairs with a genetic defect that causes chronic myelogenous leukaemia (CML). Pairs of chromosomes comprise a paternal and a maternal chromosome. From left is a normal chromosome 9, a larger defective chromosome 9, a normal chromosome 22 and a smaller defective chromosome 22. This translocational defect resulted from an exchange of material between the two defective chromosomes. The four chromosomes have replicated and will pass the defect to a new cell. The defective chromosome 22, when present in a bone marrow stem cell, leads to the increased white blood cell count in CML.