Leukaemia cells in a patient with Bloom's syndrome, coloured transmission electron micrograph (TEM). Leukaemias are a group of malignant (cancerous) diseases caused by the uncontrollable proliferation of white blood cells. Bloom's syndrome is a rare genetic disorder that, among other symptoms, causes a greater susceptibility to cancers, especially leukaemias. These cells have the typical morphology of cancer cells. The nuclei are enlarged and multilobed, the cell walls are covered in small projections and the cytoplasm contains a large number of highly active mitochondria. Magnification: x2500 when printed at 10 centimetres wide.