In polycythemia vera (PV) and essential thrombocythemia (ET) 2 specific JAK2 mutations constitutively activate the JAK-STAT pathway, explaining biologic findings such as endogenous erythroid colony (EECs) growth or PRV-1 RNA overexpression. Since these markers are detected also in JAK2 wild type patients, we hypothesized that, in these cases, the activation of the JAK-STAT pathway could be produced by a deregulation of the suppressor of cytokine signaling (SOCS) protein system. Eighty-one patients with PV and ET (53 adults and 28 children) were investigated for the methylation status of the SOCS-I, S0CS-2 and S0CS-3 CpG islands and for several myeloproliferative markers (including JAK2 and MPL mutations and clonality of hematopoiesis). SOCS-1 or SOCS-3 hypermethylation was identified in 23 patients and was associated with a significant decrease of SOCS-1 or SOCS-3 RNA and protein levels. The gene expression was restored by exposing cells to the demethylating agent 2-deoxyazacytidin. Interestingly, S1OCS-I or SOCS-3 hypermethylation was detected in 6 female patients, proved negative for JAK2 or MPL mutations and exhibiting monoclonal hematopoiesis. In conclusion, SO1CS-1 or SOCS-3 hypermethylation can activate the JAK-STAT signaling pathway in alternative or together with JAK2 mutations. These alterations might represent a potential therapeutic target. © 2008 Wiley-Liss, Inc.