Chronic myeloid leukemia (CML), also known as chronic myelogenous leukemia, is a type of cancer that starts in certain blood–forming cells of the bone marrow. In CML, a genetic change takes place in an early (immature) version of myeloid cells – the cells that make red blood cells, platelets, and most types of white blood cells (except lymphocytes). This hallmark physical change initiating CML is a reciprocal translocation involving two different genes i.e. ABL from chromosome 9 and BCR from chromosome 22 that leads to the emergence of a new abnormal fused or chimeric oncogene, BCR–ABL, on a changed chromosome 22, called the Philadelphia chromosome.