Coagulation testing and antibody studies in dog E57. (A) Canine F.IX antigen levels as a function of time after injection. Arrows denote time points at which cyclophosphamide was administered; asterisks denote infusions of canine plasma. F.IX level peaked 55 days after injection and fell rapidly after cyclophosphamide was discontinued. (B) WBCT as a function of time after injection. The WBCT fell to the normal range about 1 week after vector injection. Levels remained reduced into or near the normal range until approximately day 70 when they were again prolonged. (C) Bethesda titer as a function of time after injection. Bethesda titer was undetectable until day 77. The titer was initially low but subsequently rose to approximately 8 BU. (D) Anti–F.IX antibody subclasses as a function of time. Both subclasses remain at baseline through the duration of cyclophosphamide therapy, but IgG1 rises rapidly, and IgG2 more slowly, after cyclophosphamide discontinuation.