 This is a phase I, open-label, dose-escalation study to determine the MTD of selumetinib when combined with the standard dose of azacitidine. For the purposes of DLT assessment, subjects will be stratified into 2 cohorts- cohort A will include subjects with MDS and MDSMPN; cohort B will include subjects with myelofibrosis. Dose escalation will proceed independently in each of these cohorts. Determination of MTD will thus also proceed independently within each cohort. Three dose levels of selumetinib are planned for evaluation. Dose escalation will follow a 33 study design. Patients will be enrolled sequentially and stratified according to disease type as outlined above. An increased dose level will only open to accrual once at least 3 patients have been treated at the lower dose, followed for the defined DLT observation period 28 days, see section 2.8 below, and the lower dose level has been deemed safe. 