A 14-year-old mentally retarded girl (average mental age, 5 years) attended a special school. Her parents were healthy and were first-degree cousins. She had developed more slowly than normal and had started to talk at the age of two years. Her walking was never steady. She suffered repeatedly from febrile convulsions until the age of 11 years. She had cataracts in both eyes that were removed when she was 7 years old.
Her face was elongated with the mouth kept slightly open, but her tongue was normal in size. Her cranial nerves were normal. Her limbs were hypotrophic and hypotonic and her tendon reflexes were exaggerated. The Achilles tendons at each of her heels were enlarged, and bilateral pes cavus was noted. There was mild ataxia with unstable gait and stance. Dysarthria was present. Her sensory function was normal. Blood counts and liver function test results were normal, as were