Now, 11 years later, the diagnostic and therapeutic promise of whole-genome sequencing is finally blossoming—at least for babies. Most diseases that afflict adults are caused by a complex array of genetic and environmental factors. But genetic diseases are the leading cause of death in infants, and many are caused by a single-gene mutation. These “monogenic” diseases include well-known conditions such as cystic fibrosis, sickle-cell anemia, and Tay-Sachs disease, as well as thousands of exceedingly rare illnesses that each afflict no more than a handful to a few hundred individuals in the world. This uniqueness makes them very difficult to diagnose clinically, but because they are relatively simple genetically, they are, in theory, easy to diagnose with gene sequencing. Additionally, the cost and time involved has dropped exponentially in the past decade, and bioinformatics software has become much better at matching genetic mutations to known symptoms and conditions. It makes sense, then, that sick babies should be among the first to benefit from the technology.