One suggested reason for this apparent ESA resistance is a greater prevalence of iron deficiency: a study of anaemia in 2899 children on dialysis enrolled in the United States Renal Data System between 1996 and 2000 found that children ages 0–4 years were least likely to achieve target Hb, correlating with the lowest use of intravenous iron (33.9% versus 71%, ages 15–19) [83]. Here, GSTM1 is linked to Iron deficiency anemia.