||EXPLORING HUMAN HEREDITY 17 tions from the normal occur. The various forms of muscular dystrophy are examples of these mutations. In our laboratory we have spent much time and effort studying muscular dystrophy and other hereditary and degenerative diseases. This has been done under a U. S. Public Health grant, of which Dr. M. M. Wintrobe is the responsible investigator. It is a cooperative project where biochemists, clinicians, and geneticists work together. The biochemical studies have been carried out by Dr. Emil Smith and his associates, the clinical studies by Dr. Frank Tyler and his staff, and the genetic studies by our laboratory. Studies of the genetics of muscular dystrophy and similar studies have been done in cooperation with Dr. Tyler. To most people all muscular dystrophy is one and the same disease characterized by an atrophy or wasting away of the muscles. But it is several diseases, each being due to the action of a different gene. The three most common types are (1) Facio-Scapulo-Humeral, or FSH type; (2) Pseudohypertrophic, or childhood type; and (3) Myotonia dystrophica. The FSH type of muscular dystrophy is characterized by a wasting away of certain muscles. Weakness normally occurs first in the shoulders, face, or arms (hence the term Facio-Scapulo-Humeral muscular dystrophy). The first noticeable symptom is usually the inability to raise the arms above the head. Winged scapulae appear and dystrophy is manifested in other muscles. In cases in which the face is affected, it becomes impossible to pucker the lips, as in whistling, and there is a peculiar expression to the face when the patient attempts to smile. A flattened chest and an abnormal gait are also characteristic of the disorder. It is difficult for an affected person to raise his toes in a normal manner when he walks. He throws his foot forward and slaps it down again each time he takes a step. The dystrophy progresses with the age of the individual, sometimes continuing until late in life and in other cases halting at an early age. It occurs as often in one sex as the other. Expression of the trait ranges from very slight involvement of the muscles to a degree where the patient is almost completely incapacitated. The severe cases, however, are the exception rather than the rule. Symptoms which the patient or his family can observe usually appear between seven and twenty years of age. The most frequent age of onset is between thirteen and fifteen years.