As Hb falls so the proportion of people with microcytic anaemia increases: from 13.4% (n = 625) where Hb ≤ 11 g/dl, to 20.8% (n = 302) where Hb ≤ 10 g/dl, to a quarter of those with Hb ≤ 9 g/dl (n = 138). The gene discussed is GSTM1; the disease is anemia (phenotype).