Preexisting transient IgG antibodies binding to PEG-FVIII, and treatment-emerging transient IgG antibodies binding to FVIII. The patient (patient 24) participated in studies NCT02210091 (study A) and NCT01945593 (study B). They are 1 of 2 patients who presented with preexisting IgG antibodies binding to PEG-FVIII and, in addition, developed IgG antibodies binding to FVIII after exposure to rurioctocog alfa pegol. Both antibody populations were transient and disappeared over the course of the clinical studies.