Background:: Fragile X syndrome (FXS) is the most common cause of inherited intellectual disabilities and autism. The reported prevalence of the full mutation (FM) gene FMR1 in the general population is 0.2–0.4 per 1000 males and 0.125–0.4 per 1000 females. Population screening for FMR1 expanded alleles has been performed in newborns and in an adult population. However, it has never been carried out in an entire town. Ricaurte is a Colombian district with 1186 habitants, with a high prevalence of FXS, which was first described by cytogenetic techniques in 1999. Methods:: Using a PCR-based approach, screening for FXS was performed on blood spot samples obtained from 926 (502 males and 424 females) inhabitants from Ricaurte, accounting for 78% of total population. Results:: A high prevalence of carriers of the expanded allele was observed in all FXS mutation categories. Using the Bayesian methods the carrier frequency of FM was 48.2 (95% Credibility Region CR: 36.3–61.5) per 1000 males and 20.5 (95% CR:13.5–28.6) per 1000 females; the frequency of premutation carrier was 14.1 (95% RC: 8.0–21.7) per 1000 males (95% RC: 8.0–21.7 per 1000 males) and 35.9 (95% RC: 26.5–46.2) per 1000 for females (95% RC: 26.5–46.2 per 1000 females), and gray zone carrier was 13.4 (95% RC: 7.4–20.7) per 1000 males (95% RC: 7.4–20.7 per 1000 males) and 42.2 (95% RC: 32.2–53.8) per 1000 for females (95% RC: 32.2–53.8 per 1000 females). Differences in carrier frequencies were observed for premutation and FM alleles between natives and non-natives. Conclusions:: This study shows that in Ricaurte the carrier frequencies of FMR1 expanded alleles (premutations and FMs) are higher than those reported in the literature, suggesting that Ricaurte constitutes a genetic cluster of FXS.
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