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Myotonic dystrophy type 1 (DM1) is a multisystem disorder with progressive myopathy and myotonia. The clinical study was conducted in the Republic of North Ossetia-Alania (RNOA), and in it 39 individuals from 17 unrelated families were identified with DM1. Clinical presentations varied, including muscle weakness, fatigue, intellectual disability, hypersomnia, ophthalmological abnormalities, and alopecia. Using clinical and genotyping data, we confirmed the diagnosis and enabled the study of CTG-repeat anticipation and DM1 prevalence in the Ossetian and Ingush populations. CTG expansion correlated with age of onset, with clinical severity, and with offspring showing more severe symptoms than parents. In many families, the youngest child had a more severe DM1 phenotype than older siblings. The prevalence was 14.17 per 100,000 in Ossetians and 18.74 per 100,000 in Ingush people, aligning with global data. Segregation analysis showed a higher frequency of maternal transmission. The study highlights the clinical and genetic heterogeneity of DM1 and its dependence on repeat expansion and paternal and maternal age. [ABSTRACT FROM AUTHOR]