The enormous progress in the development of drugs for rare diseases may be attributed to advances in genomic technology, molecular profiling, improved target and biomarker selection, an improved understanding of the natural history and pathophysiology of several orphan diseases, use of integrated quantitative analysis techniques in drug development, and a favorable regulatory climate, but major challenges still remain. Most rare diseases manifest during childhood; about 30% of affected children die before their fifth birthday, and the health and economic burden on survivors can be tremendous.