This is a phase 2, crossover study of Ataluren for the treatment of nonsense mutation Dravet syndrome or cyclin-dependent kinase-like 5 (CDKL5) deficiency, resulting in drug-resistant epilepsy. Patients will receive 12 weeks of ataluren or placebo during each treatment period. Treatment Period 1 will be followed by a 4-week Washout Period. Based on ataluren PK and pharmacodynamic data, the 4-week washout period is deemed an appropriate length of time to eliminate any ataluren drug effects. Following the Washout Period, patients will crossover to receive the opposite treatment during Treatment Period 2 as follows: Patients receiving ataluren during Treatment Period 1 will receive placebo during Treatment Period 2. Patients receiving placebo during Treatment Period 1 will receive ataluren during Treatment Period 2.