Fragile X syndrome is caused by dynamic mutation of FMR1 gene CpG island CGG repeats. The underlying mutational mechanism is not fully understood. Different microsatellite markers and SNP have previously been reported as markers associated with FMR1 CGG repeat instability. The aim of the present study was to identify specific haplotypes among Latvian FXS patients and the control group with respect to allelic stability. Eleven male FXS patients and 122 control male patients participated in the study. In total, 27 different DXS548-FRAXAC1-ATL1-FRAXAC2 haplotypes were found. The prevalent haplotype in the control group was 7-4-A-5+ (rel. frequency 0.327). The prevalent haplotype associated with the FXS group was 2-2-G-4 (rel. frequency 0.818; p < 0.0001). Grey zone alleles with a long uninterrupted CGG tract at the 3’ end were significantly associated with the 2-2-G-4 haplotype (p = 0.0022). Our findings suggest that, for the Latvian population, the haplotype 2-2-G-4 is a marker of CGG tract instability. We conclude that a founder effect could not be an explanation for our findings on the basis of heterogeneity exhibited by the Latvian population and lack of studies throughout this geographical region. This data may provide evidence of different mutational pathways of expansion in the Baltic States region.