Bosutinib is a 2nd generation tyrosine kinase inhibitor that has shown promising results from first up to fourth line treatment in patients with in chronic phase of chronic myelogenous leukaemia. Most patients discontinuing the treatment with Bosutinib do so because of side effects occuring early after starting the treatment. A step in dosing scheme could improve these early toxicities. The aim of this study therefore is to demonstrate that temporary lowering of the Bosutinib dose during early treatment may help to reduce or prevent side effects while preserving efficacy.