The Philadelphia chromosome is a genetic abnormality found on chromosome 22 of chronic myeloid leukemia (CML) patients. The abnormality arises due to the reciprocal translocation between chromosome 22 and chromosome 9 (t(9;22)(q34;q11)) and a fused gene called BCR-ABL1. The translocation is also known as the Philadelphia translocation (Ph).
The ABL1 gene encodes a protein called Tyrosine-protein kinase ABL1 is located on chromosome 9. The BCR gene encodes a protein called renal carcinoma antigen NY-REN-26 is located on chromosome 22. Due to the translocation, a fusion BCR-ABL1 gene is formed, which encodes constitutively active tyrosine protein kinase. This induced the cell to divide uncontrollably by interrupting the signaling pathways that regulate the cell cycle.