The pathogenesis of Fragile-X syndrome is a consequence of absence of the FMR1 gene product associated with expansion of the CGG repeat and abnormal methylation of this and a CpG island 250 bp proximal to the CGG repeat located at exon 1 in the FMR1 gene. While this is usually the case, some suspected Fragile-X syndrome patients have been described with a mutation other than CGG expansion. We describe here an affected Fragile-X male, who was found to be mosaic of a full mutation of the CGG expansion and a deletion in the FMR1 gene. The patient's phenotype is probably mainly due to the effect of the full mutation of the repeat sequence. Thus, the influence of the deletion is difficult to evaluate.