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How the Philadelphia chromosome forms
An abnormal chromosome called the Philadelphia chromosome is associated with chronic myelogenous leukemia. Your cells each contain 23 pairs of chromosomes that are made of DNA and hold the instructions for every cell in your body. The Philadelphia chromosome forms when chromosome 9 and chromosome 22 break and exchange portions. This creates an abnormally small chromosome 22 and a new combination of instructions for your cells that can lead to the development of chronic myelogenous leukemia.