RBC, red blood cell; Hb, hemoglobin; Hct, hematocrit; MCV, mean corpuscular volume; Epo, erythropoietin; NA, not available; ND, not done; HBT, heterozygous beta-thalassemia; PV, polycythemia vera. The gene discussed is GSTM1; the disease is beta thalassemia.