Relative levels of intracellular and secreted hFIX of myotubes in comparison with those of myoblasts. Arrow indicates the time point of switching to the differentiation medium. Results shown are averages of two independent experiments with observed ranges shown in vertical bars. hFIX expressed by myoblasts at day 2 are 7.7 ± 0.1 ng/10⁶ cells and 593.2 ± 8.9 ng/10⁶ cells/24 hours for intracellular (□) and secreted hFIX (▪) from cells stably transfected with pdLMe4βAhIXm1, 6.2 ± 2.1 ng/10⁶ cells and 581.6 ± 74.9 ng/10⁶ cells/24 hours for intracellular (○) and secreted hFIX (•) from myoblasts transduced with LIXSN, respectively. Intracellular and secreted hFIX levels of myotubes were normalized to those of myoblasts (day 2), which were defined as 1 as shown on the Y-axis.