GSTM1 and polycythemia: This is particularly relevant in Chronic Mountain Sickness (CMS), a clinical syndrome commonly presented in high-altitude natives and life-long residents, which frequently coincides with excessive erythrocytosis (Hb ≥ 19 g/dL for women, Hb ≥ 21 g/dL for men) in some high altitude groups (León-Velarde et al., 2005; Oberholzer et al., 2020).