The Philadelphia chromosome is an abnormal chromosome found in some cancer cells, particularly in patients with chronic myeloid leukemia (CML). It results from a genetic mutation where parts of chromosomes 9 and 22 swap places, creating a fusion gene called BCR-ABL. This fusion gene produces a protein that promotes uncontrolled cell growth. The presence of the Philadelphia chromosome is a key factor in diagnosing CML and can influence treatment decisions. Targeted therapies, such as imatinib, specifically aim to inhibit the activity of the BCR-ABL protein, helping to manage the disease more effectively.