PROJECT NARRATIVE Though any single rare disease is by definition uncommon, together these diseases affect nearly 30 million individuals in the United States, two-thirds of whom are children, and contribute significantly to morbidity, mortality, and healthcare costs. Exome and whole genome sequencing have the potential to provide a diagnosis to an estimated 25 to 50 percent of those patients struggling with undiagnosed rare diseases, but the downstream costs and benefits of this testing beyond diagnosis remain largely unknown.