MPL and myeloproliferative neoplasm: In contrast, cells that harbor STAT5-activating mutated or modified oncoproteins, like BCR-ABL, JAK2 V617F, mutant MPL (thrombopoietin receptor), and mutant calreticulin in myeloproliferative neoplasms will exhibit persistent activation of STAT5, with high constant levels in the nucleus [40].