We report on a family segregating a FMR-1 allele within the "grey zone" of triplet repeat length (n = 51). The allele showed a 1-unit increment when transmitted through a female meiosis and a 1-unit increment when transmitted through a male of the next generation. At the following generation, a pregnant woman had amniocentesis performed. The latter showed she transmitted the allele unchanged (n = 53) to her male fetus. This family was not ascertained through an affected subject, and there was no family history of mental retardation. Thus our observation reflects the natural history of an unstable allele in the general population. Systematic analysis of such alleles may help refine our understanding of the grey zone of triplet repeat length.