Background: In a phase 2 trial of peg-IFN-alpha-2a, we observed disappearance of JAK2V617F in peripheral blood (PB) granulocytes in 7/29 (24%) treated PV patients using an allele specific PCR assay with a 1% sensitivity (Blood online, July 23, 2008). IFN-alpha has been shown to induce cytogenetic remission or reversion to polyclonal hematopoiesis by studies of occasional patients. The 7 cases of complete molecular remissions (CMR) we identified by V617F monitoring constitute the first cohort of patients with measurable reduction of the PV clone. The study of this cohort may allow answering whether such CMR are associated with disappearance of the PV clone. Methods: Bone marrow (BM) was collected at time of CMR after 12 to 30 months of peg-IFN-alpha-2a in 5 V617F-positive (pos) patients. We studied erythroid colonies generated in the presence or in the absence of erythropoietin (Epo) from total BM mononuclear cells in standard plasma clot (SPC) assay, and purified BM CD34+ cells in methylcellulose (MC) assay. Individual erythroid colonies grown from BM CD34+ cells were harvested and screened for V617F. Results are expressed as mean of multiple assays (2 to 6, according to the number of cells available). Results: At time of CMR, SPC assays revealed the presence of few residual endogenous erythroid colonies (EEC) in the 5 patiens, representing only 0.2% to 3.6% of the total number of colonies grown with Epo (table 1, column 1). MC assays (from BM CD34+ cells) similarly showed few EEC in 4 patients (n°1, 3, 4, 5), representing 2.4 to 6% of colonies grown with Epo, while in one patient (n°2) no EEC was detected (column 2). In 2 of the 4 patients with persisting EEC in MC assays (n°1, 4, column 3), all EEC (n=5 and 26) were V617F-neg, while in the other 2 (n°3, 5) V617F was found in 4/9 and 1/28 EEC, respectively. In the patient with no EEC (n°2), V617F was however detected in 1 of 64 colonies grown with Epo (column 4). V617F was also found in 1.2% to 5.7% of the colonies grown with Epo in 3 pts (n° 3, 4, 5, column 4), while in the last patient (n°1) with V617F-neg EEC, colonies grown with Epo were also completely V617F-neg. Conclusion: In 1 of 5 V617F-positive PV patients who achieved complete disappearance of the mutation in peripheral blood after peg-IFN-alpha-2a (patient n°1 in table 1), no V617F-positive marrow erythroid progenitors were found, either after EPO stimulation or in EEC, while frequency of V617F-positive marrow progenitors was reduced to very low levels in the other patients. This supports the hypothesis that peg-IFN-alpha-2a can eradicate the JAK2-mutated clone in at least a few selected patients, and greatly reduce it in others. In addition, the finding that a few EEC (mostly V617F-negative) were still found in 4/5 patients analyzed, suggests that a subset of erythroid progenitors from those patients retained the ability to form EEC by a mechanism independent of JAK2V617F.