Even for the previously reported inter-cell mosaicism at the FMR1 locus [3], Southern hybridization does not readily detect mosaicism in those instances where there is a broad size distribution of unmethylated epialleles, and could thus obscure inter-cell mosaicism at FMR1. We reasoned that if methylation mosaicism were the basis for the observed FMR1 expression, then double-stranded DNA methylation patterns of the FMR1 promoter might reveal mosaicism of an unusual form not previously assessed in FXS. This evidence concerns the gene FMR1 and fragile X syndrome.