Break of immune tolerance against human FVIII in mice of subline E is not isotypically restricted. Transgenic mice expressing human F8 cDNA (huF8, A-C) and conventional hemophilic E17 F8-knockout mice (E17F8ko, D-F) were treated weekly with 9 doses of either native human FVIII (FVIII, A and D) or 1 of 2 PEGylated human FVIII proteins (mFVIII1, B and E; mFVIII2, C and F). Subclasses of IgG, IgA, and IgM antibodies specific for native human FVIII were analyzed by ELISA 1 week after the last treatment. Results of individual mice are shown.