To most people, Q93.51 would seem like an arbitrary collection of letters and numbers. But for Terry Jo Bichell, it represents an important victory, and the hope of something better for her son Lou. Lou, 19, is the youngest of five siblings, and the only one born with Angelman syndrome —a genetic condition characterized by a happy, excitable demeanor, but also by absent or minimal speech, delayed development, movement problems, and a high risk of seizures. It’s estimated that between 1 in 12,000 to 1 in 20,000 people have the condition, although exact figures are hard to come by since many are misdiagnosed with autism or cerebral palsy. In the past, when Lou and other Angelman patients went to see their doctors, they would be listed under the medical code Q93.5. That code and others like it come from the International Classification of Diseases —a master list of health problems, as defined by the World Health Organization. These codes might seem like bureaucratic arcana, but th...