Background: Transcription factor Stat5a/b is highly critical for the viability of human prostate cancer cells in vitro and for prostate tumor growthin vivo. Stat5 is constitutively active in clinical prostate cancers but not in the normal human prostate epithelium. Moreover, Stat5a/b activation in prostate cancer is associated with high histological grade of prostate cancer. However, the molecular mechanisms underlying constitutive activation of Stat5a/b in prostate cancer are unclear. The receptor-associated tyrosine kinase Jak2 is a known key activator of Stat5a/b in prostate cancer cells in response to ligand stimulation. Recently, a single gain-of-function point mutation ofJAK2was described in myeloproliferative diseases leading to constitutive Jak2 kinase activity, subsequent Stat5a/b activation and involvement of V617F Jak2 in the pathogenesis of myeloproliferative disorders.Materials and Methods: We determined whetherJAK2undergoes the V617F activating mutation during clinical progression of human prostate cancer using a highly sensitive assay (amplification refractory mutation system) and a unique material of fresh specimens from organ-confined or castration-resistant prostate cancers.Results: TheJAK2V617F mutation was not found in any of the normal or malignant prostate samples analyzed in this study.Conclusions: Future work should focus on determining the molecular mechanisms other than V617F mutation of Jak2 resulting in continuous Stat5 activation in clinical prostate cancers.