Results from clinical gene therapy for hemophilia with AAV expressing factor IX (FIX) indicate that the development of capsid specific T-cell response is dependent on the dose of the vector (Manno et al., 2006; High & Aubourg, 2011; Mingozzi & High, 2011,2013; George & Fogarty, 2016). The gene discussed is F9; the disease is hemophilia.