Chronic myeloid leukaemia is a rare type of cancer affecting approximately 700 people (mostly adults) in the UK each year.

Leukaemia is a cancer of the white blood cells. Normally, white blood cells grow and divide in an orderly and controlled way, but in leukaemia the process gets out of control and the cells divide too quickly, and do not mature. In CML, too many myeloid cells (one of the main types of white blood cells) are produced. The myeloid cells are released into the blood when they are immature and unable to work properly. These immature white blood cells are known as blasts.

The immature cells fill up the bone marrow and prevent it from making blood cells properly. As the leukaemia cells do not mature, they can't do the work of normal white blood cells, which leads to an increased risk of infection. Because the bone marrow is overcrowded with immature white cells it also can't make enough healthy red cells and platelets.

CML usually develops very slowly, which is why it is called ‘chronic’ myeloid leukaemia.

Chronic myeloid leukaemia can occur at any age, but it more commonly affects middle-aged and older people. It is rare in children.