A new kind of testing is proving particularly helpful in diagnosing mysterious neurological illnesses in children. Scientists sequence all of a patient’s genes, systematically searching for disease-causing mutations. A few years ago, this sort of test was so difficult and expensive that it was generally only available to participants in research projects like those sponsored by the National Institutes of Health. But the price has plunged in just a few years from tens of thousands of dollars to around $7,000 to $9,000 for a family. Baylor College of Medicine and a handful of companies are now offering it. Insurers usually pay.