The discovery of the Fragile X (fra(X] syndrome represents a major advance in our understanding of mild mental retardation. This X-linked syndrome is the most common hereditary form of mental retardation. Recent estimates find that approximately 1/981 males and 1/677 females carry the fra(X) chromosome. The majority of affected males are moderate to severely retarded, but about 20% are mildly retarded and about 5% are borderline. Approximately 20% of males who inherit the fra(X) chromosome are termed non-penetrant; they do not express it cytogenetically and are of normal intellect. About 1/3 of carrier females show mental impairment and about 10% are mildly retarded. We have found evidence for genetic heterogeneity based on linkage analysis to flanking DNA probes. Some large families show tight linkage between fra(X) and the flanking probe F9, while others show loose linkage. Preliminary findings indicate the linkage heterogeneity may also be related to cognition: affected males in tightly linked families tended to be mildly retarded.