A possible explanation could be seen in the high number of patients with relatively mild anemia (less than 2 g/dl Hb below 3rd percentile), but even when looking at severe anemic patients alone, the proportion of patients that receive iron supplementation only increases slightly to 41% in UC, 22.2% in CD, and 20.4% in IBD-U. The gene discussed is GSTM1; the disease is anemia (phenotype).