Although recombinant protein replacement therapies in several diseases, particularly Pompe and to a lesser extent haemophilia, have been shown to have a negative clinical response to inhibitory antibody induction [32], this has not been the case in MPS I. 91% of MPS I patients treated with laronidase on the original ERT trial developed IgG or IgM allo-antibodies to recombinant enzyme [16], although no evidence of poor clinical outcome has been demonstrated in MPS until now. Here, CD40LG is linked to hemophilia.