The recent discovery of calreticulin mutations in myeloproliferative neoplasms point to the unexpected role of the endoplasmic reticulum biology in the pathophysiology in these diseases. Otherwise, the association of endoplasmic reticulum stress with solid cancers, in particular in resistance to chemotherapy, is well documented, contrary to hematological malignancies. The study aims to evaluate endoplasmic reticulum stress markers as predictors for the response to hydroxyurea in polycythemia vera and essential thrombocythemia patients. The main objective is to correlate endoplasmic reticulum stress (defined as splicing of XBP1 above 30%) to the rate of complete response after 6 months according to the 2009 ELN criteria. This is an observational retrospective study.