Sir.—Based on their study and the case reports of others, Hagerman et al1 have urged that fragile X (fra[X]) syndrome be considered in the differential diagnosis of children with learning disabilities. Fragile X syndrome is the most common form of familial mental retardation.1 Since it is carried on the X chromosome, it is expected to affect boys more frequently and more severely than girls. The frequency in girls, however, is about four fifths of that in boys, which is probably due at least in part to lyonization.2 Fragile X syndrome is characterized in male patients by mental retardation, large ears (macrotia), and macro-orchidism. Some patients, however, present with learning disabilities (LD). I should like to second the recommendation of Hagerman and colleagues that fra(X) syndrome be considered in the differential diagnosis of patients with LD for two theoretical reasons. One of these is that biologic age does