Leukaemia, conceptual composite image. Scanning electron microscopy (SEM) and computer generated image (CGI) of DNA (deoxyribonucleic acid) molecules (helices) and red and white blood cells. Leukaemia is cancer of the white blood cells. Leukemias are caused by mutations in the DNA of blood cells. These genetic mutations change the way the blood cells reproduce and can also prevent them from functioning properly. Eventually, the abnormal blood cells crowd out healthy blood cells and can block the bone marrow from producing more healthy cells. The mutations are genetic, but usually not hereditary. This means that while leukemia is caused by mutations in your genes, these genetic abnormalities aren't often inherited from your family. This is called an acquired gene mutation.