Tyrosine kinase and bosutinib complex. Computer illustration showing a molecule of bosutinib (spheres) bound to a tyrosine kinase molecule (ribbons). Tyrosine kinase is an enzyme that can transfer a phosphate group from ATP (adenosine triphosphate) to a protein in a cell. Bosutinib is a Bcr-Abl tyrosine-kinase inhibitor (TKI), a group of TKIs that inhibit tyrosine kinase associated with the Philadelphia chromosome (Bcr-Abl). Bcr-Abl TKIs are the first-line therapy for most patients with chronic myelogenous leukaemia (CML). Over 90 percent of CML cases are caused by a chromosomal abnormality that results in the formation of a so-called Philadelphia chromosome.