Philadelphia chromosome. Coloured light micrograph of a female karyotype (chromosome set) with one defective chromosome each from pairs 9 and 22. The defects, on the right-hand chromosomes of the two pairs, cause chronic myelogenous leukaemia (CML). The 46 chromosomes of a human karyotype are here arranged in 23 pairs of a maternal and paternal chromosome. The left-hand chromosomes of pairs 22 and 9 are normal, whereas an exchange of material (translocational defect) made defective chromosome 9 larger and shrunk defective chromosome 22. In a bone marrow stem cell, the chromosome 22 defect causes the CML increased white blood cell count.