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Background: The STAG1 gene encodes a component of the cohesin complex, involved in chromosome segregation and DNA repair. Variants in genes of the cohesin complex determine clinical conditions characterized by facial dysmorphisms, upper limb anomalies, intellectual disability, and other neurological deficits. However, to date, the STAG1-related clinical phenotype has been poorly investigated (around 20 cases reported). Methods and Results: We report, for the first time, two twins affected by a syndromic neurodevelopmental disorder associated with a de novo variant in the STAG1 gene. Although both the twins showed a neurodevelopmental delay, one of them showed a more severe phenotype with greater behavioral problems, speech defects and limb apraxia. CGH array showed a 15q13.3 microduplication, inherited from an unaffected mother. Conclusions: We found different degrees of behavioral, speech and cognitive impairment in two twins affected by a neurodevelopmental disorder associated with a STAG1 variant. These findings highlight the variability of the STAG1-associated phenotype or a probable role of associated variants (like the discovered 15q13.3 microduplication) in modulating the clinical features. [ABSTRACT FROM AUTHOR]