MEK111759 is a dose-escalation, Phase I/II, open-label study to determine the recommended dose and regimen for the orally administered MEK inhibitor GSK1120212 in subjects with relapsed or refractory leukemias. The recommended dose and regimen will be selected based on the safety, pharmacokinetic, and pharmacodynamic profiles. This study will identify the maximum tolerated and recommended Phase II doses using a dose-escalation procedure. Dose escalations will continue based on predefined parameters until a maximum tolerated dose is established. In Phase II, the clinical efficacy of GSK1120212 in subjects with relapsed or refractory leukaemias (AML, MDS or CMML) will be determined.