GSTM1 and anemia: Several potential confounders were not measured or recorded in the parent study: the number and age of transfused packed red blood cells [44,45], pre- and postoperative Hb levels, iron and Hb status before and after BT [3], the management of preoperative and postoperative anemia through high-dose i.v. iron preparations [46,47], and, as reported above, the composition of blood donors and recipients’ microbiome [48].