More than 90% of polycythemia vera (PV) patients have a mutation in the protein JAK2,which is closely associated with the erythrocyte membrane. With the comparison of 1-D gelsof erythrocyte membranes obtained from PV patients treated with hydroxycarbamide andthose of untreated controls we observed significant differences in the region of 40–55 kDa.On the basis of the LC–MS/MS analysis of this region we report up-regulation of four proteindisulfide isomerases, which was subsequently confirmed by targeted mass spectrometricanalysis. In further studies it will be prudent to compare this in patients both treated andnot treated with hydroxycarbamide.