Adequacy of tissue oxygenation (tissue PO2) is judged by mixed venous PO2 (PvO2).[22, 23] Normal values of PvO2 is 35-45 mm Hg (5-6 KPa), which corresponds to a normal mixed venous O2 content difference CvO2 of 12-15 vol% and a mixed venous Hb saturation SvO2 of 72-78%.[23–25] Thus, we can deduct that as PvO2 falls from 40 - 47 in normal patients to 26 mm Hg in severe anaemia [Figure 1], tissue hypoxia may occur. The gene discussed is GSTM1; the disease is anemia.