MYT1L syndrome is a rare genetic syndrome, recently described in 2011, with paediatric onset, responsible for a neurodevelopmental disorder combining psychomotor retardation, learning difficulties and/or intellectual development disorders, epilepsy, overweight and eating disorders. The Rouen genetics department is currently positioned as a clinical expert in this disease. The study published in 2020 by our team (Coursimault J et al., Hum Genet. 2022, PMID: 34748075) has enabled us to describe 40 new individuals worldwide, to gain a better understanding of this disease, to specify the genotype-phenotype relationships and to describe new clinical signs. We were able to confirm the presence of a neurodevelopmental disorder in 100% of patients, which includes: language delay, impaired orality, global and facial hypotonia, prosodic features and behavioural problems. This will be the first study in the world to characterise the neuropsychological, language and prosodic profiles of MYT1L patients.