Furthermore, it is known that expansion of both PV and RBCV can occur with polycythemia vera and thus present normal Hb and HCT, so-called masked polycythemia.[17] To account for this occurrence, the Hb and HCT thresholds for considering polycythemia have been lowered to 16 g/dL and 48% for women and 16.5 g/dL and 49% for men.[18] These cutoff values overlap considerably with normal ranges at high altitudes, underscoring the need for better screening methods to select individuals for polycythemia evaluation at high altitudes. Here, GSTM1 is linked to acquired polycythemia vera.