Hemophilia A (HA) and hemophilia B (HB) are inherited bleeding disorders caused by mutations in the gene for factor VIII (FVIII) and factor IX (FIX), respectively. These proteins are essential for blood clotting. The lack of FVIII/FIX can produce bleeding episodes that cause damage of the bone, muscles, joints, and tissues. A specific type of mutation, called a nonsense (premature stop codon) mutation, is the cause of the disease in approximately 10-30% of patients with hemophilia and results in severe manifestations. Ataluren (PTC124) is an orally delivered, investigational drug that acts to overcome the effects of the premature stop codon, potentially enabling the production of functional FVIII/FIX. This study is a Phase 2a trial evaluating the safety and efficacy of ataluren in patients with HA or HB due to a nonsense mutation. The main purpose of this study is to understand whether ataluren can safely increase FVIII/FIX activity levels.