When a baby is slow to crawl or talk or has other symptoms suggesting a genetic disorder, parents often find themselves embarking on a long, frustrating quest for answers. Doctors may order a series of tests but cannot arrive at a specific diagnosis. Now, cheap DNA sequencing could help uncover the causes of such mystery disorders. The idea is to sequence the 1% of the patient’s DNA that codes for protein—the “exome”—then sift the data for the genetic culprit behind the disease. Today in The New England Journal of Medicine, researchers report that using exome sequencing, they successfully identified the mutations underlying the conditions of 25% of 250 patients, most children with neurological disorders. Among them were three patients with Cornelia de Lange syndrome, which can cause intellectual delays and distinctive facial features such as long, thick eyebrows and downturned lips (above). This success rate suggests that exome sequencing could be used routinely for clinical diagnosis of a variety of rare genetic conditions.