In analysis of ex vivo colony forming assays from ET and PV patients, transcriptional analysis demonstrated an enrichment of STAT5A targets with nuclear phosphorylation of STAT5A identified in JAK2 V617F position colonies from both ET and PV patients but not wild type colonies whilst a recent phospho-proteomics approach identified STAT5 and STAT3 as differentially phosphorylated in JAK2 V617F mouse haematopoietic cells [58, 59]. The gene discussed is STAT5B; the disease is acquired polycythemia vera.