This 10-year-old boy was brought to medical attention by his mother with complaints of significant incoordination throughout childhood and notable weakness in the last 2 years. In particular, the mother noted that he had difficulty climbing stairs and needed to use the railing. He had been receiving individual attention from his physical education teacher at school for 2 years because of slow and uncoordinated running. Academically, he was doing very well, with good handwriting and drawing skills. The child was the product of a full-term pregnancy and spontaneous natural delivery. He weighed 3864 g at birth. He rolled over at age 4 months, sat without support at age 6 to 7 months, and walked at age 14 months. His first word phrases were at age 15 months, and his first sentences, at age 2½ years. The mother thought he was less active prenatally than his older brother.
Family history was