Glivec leukaemia drug binding to its target, the enzyme Bcr_Abl tyrosine kinase. This enzyme is produced by a genetic mutation, the swapping of parts of two chromosomes, leading to the formation of the oncogene cancer_causing gene Bcr_Abl, which produces a constantly_active version of normal Abl tyrosine kinase. This causes chronic myelogenous leukaemia CML, a cancer of white blood cells. The drug Glivec Imatinib works by binding to the active site of the Bcr_Abl enzyme, producing a change in shape that prevents the enzyme from functioning. This causes the death of cancerous cells expressing the enzyme. Glivec, the first such tyrosine kinase inhibitor to be produced, is marketed by Novartis.