CML is a cancer of white blood cells (WBCs), which are part of the immune system responsible for fighting infections.
Under normal circumstances, the body carefully regulates the number of white blood cells circulating in the blood, increasing them when needed to fight an infection and bringing them back to normal when the infection is controlled.
In CML, this regulation is lost due to over production of abnormal WBCs in the bone marrow. The abnormal WBCs fill up the bone marrow and spill into the blood, resulting in symptoms of CML.
This form of leukemia is called chronic because it frequently follows an indolent course, slowly building up over months or years. This is different from acute myelogenous leukemia, which is caused by different mutations, and typically following a rapid, aggressive course. There are 3 phases of CML. The first one is called the chronic phase, and follow an indolent course. However, some patients with CML move on to progressively more aggressive phases, called the accelerated phase and blast phase, which are associated with more symptoms, are harder to treat and are a greater threat to life.
Fortunately, the chronic phase of CML can now be successfully treated with target agents known as tyrosine kinase inhibitors (TKIs) of BCR-ABL1, and many patients can expect to live many years to a normal life expectancy.