A model for the myeloproliferative diseases.JAK2-positive polycythemia and thrombocythemia overlap and can progress to an accelerated phase. This can be clinically variable, with myelofibrotic transformation, cytopenias, increased blasts, and increased white cells potentially present. JAK2-negative disease follows similar patterns of progression and is biologically distinct from V617F-positive disease. Under this model, patients currently labeled as having primary myelofibrosis may in fact represent persons in the accelerated phase of a preexisting MPD.