For instance, one of the XOR inhibitors, oxypurinol, did not improve clinical outcomes in unselected patients with moderate-to-severe HF in the OPT-CHF (The Efficacy and Safety Study of Oxypurinol Added to Standard Therapy in the Patients With New York Heart Association Class III-IV Congestive Heart Failure) study [135]. This evidence concerns the gene XDH and hydrops fetalis.