A common point mutation in the JAK2 tyrosine kinase leads to constitutive hematopoietic growth factor receptor signaling and was recently described in many patients with myeloproliferative disorders (MPDs). However, this JAK2 mutation is present in only a subset (35-50%) of patients with essential thrombocythemia (ET). Thus, the proliferative signals responsible for MPDs in the absence of JAK2 mutations remain largely unknown. Despite intriguing pre-clinical data, where transgenic mice overexpressing FLT3-ITD developed a MPD resembling ET, none of the patient samples from ET patients who were JAK2(V617F)-negative demonstrated the presence of activating mutations in the FLT3 receptor.