Fragile X syndrome (FXS) has a classic phenotype, however its expression can be variable among full mutation males. This is secondary to variable methylation mosaicisms and the number of CGG triplet repeats in the non-coding region of the Fragile X Mental Retardation 1 ("FMR1") gene, producing a variable expression of the Fragile X Mental Retardation Protein (FMRP). Here we report a family with several individuals affected by FXS: a boy with a hypermethylated "FMR1" mutation and a classic phenotype; a man with an "FMR1" gene mosaicism in the range of premutation (PM) and full mutation (FM), who has a mild phenotype due to which FXS was initially disregarded; and the cases of four women with a FM and mosaicism. This report highlights the importance of DNA molecular testing for the diagnosis of FXS in patients with developmental delay, intellectual disability and/or autism due to the variable phenotype that occurs in individuals with "FMR1" mosaicisms.