An inherited disease characterized by the presence of a fragile site in the long arm of chromosome X. It is a common cause of mental retardation, second only in frequency to the Down syndrome (trisomy 21). The expression varies with mental retardation, macroorchidism, high-pitched voice, narrow face, long jaw, large ears, prominent forehead, highly arched narrow palate, and joint laxity as the most common characteristics. Microcephaly, typical facies, shortness of stature, and absence of macroorchidism characterize the Renpenning but not Martin-Bell syndrome. Major characteristics of the Martin-Bell syndrome include: Mental retardation with speech and behavioral disorders; connective tissue dysplasia; square facies with midfacial hypoplasia; slightly below normal height without intra- uterine growth retardation; average or above average head circumference; large and frequently anteverted ears; prominent forehead and supraorbital ridges; large nose; prominent mandible which becomes apparent during adolescence; joint laxity; minor limb anomalies; dermatoglyphic abnormalities; and seizures.