Abstract
The fragile site at Xq27 which is associated with X-linked mental retardation with macroorchidism has been studied in 21 retarded males. These males were from 12 families, and studies of nine of the families were possible. Detection of carrier females is difficult, especially with increasing age. The fragile site was demonstrated in only five of 13 obligate carrier females. It is concluded that using present methods, cytogenetic detection of carriers is fairly reliable in females aged less than 20–25 years but unreliable in older females.