Frequency of fragile X chromosome in normal females

Because of the ambiguities in diagnosing carriers of the fragile X syndrome, we studied thirty‐six normal females to determine whether the fragile site at Xq27 can be seen in noncarrier females and at what frequency. A fragile site at Xq27 was identified in one out of thirty‐six females, occurring at a frequency of 0.5% in her peripheral lymphocytes. We conclude that the fragile Xq27 site occurs only rarely in noncarrier females and that each laboratory should determine its own baseline frequencies of fragile X in order to most accurately distinguish between normal and carrier women.