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Milena Barbosa Porto,1 Geovanna da Mata e Castro,1 Samara Socorro Silva Pereira,2 Elza Maria Gonçalves Santos Uchoa,2 Raffael Zatarin,3 Lysa Bernardes Minasi,1 Aparecido D da Cruz1– 3 1Graduate Program in Genetics, School of Medical and Life Sciences, Pontifical Catholic University of Goiás, Goiânia, GO, Brazil; 2Federal University of Goiás, Graduate Program in Genetics and Molecular Biology, Goiânia, GO, Brazil; 3Clinical Genetics Service, Center for Rehabilitation and Readaptation Dr. Henrique Santillo, State Health Secretary of Goiás, Goiânia, GO, BrazilCorrespondence: Aparecido D da Cruz, Email acruz@pucgoias.edu.brAbstract: Baker-Gordon Syndrome (BAGOS) is a genetically determined 4 (NDD), represented by a phenotypic spectrum of moderate to severe intellectual disability, resulting from mutations in the synaptotagmin 1 (SYT1) gene. Its prevalence is estimated at 1:1,000,000 and the known gene variants have indicated complete penetrance with variable expressivity. SYT1 is a membrane trafficking protein in presynaptic vesicles, which exerts a complex influence on synaptic transmission, with fundamental roles in the release of neurotransmitters and facilitators of endocytosis, impacting both neurotransmission and neuron plasticity. The current case report describes the first Brazilian male patient diagnosed at 17-year-old, and the 39th reported case globally using whole-exome sequencing. A de novo heterozygous missense mutation at chr12q:79448958 (NM_005639.2; c.1103T>C; p.Ile368Thr) in the SYT1 was found and classified as a pathogenic variant. The proband’s clinical phenotype was compatible with BAGOS, involving behavioral changes such as irritability and severe intellectual disability. Knowledge about the mechanism of action and the extent of the genotypic and phenotypic presentations of the mutations in the SYT1 is still unfolding. Thus, we aimed to describe additional genotype–phenotype correlation for BAGOS, contributing to the expansion of the existing knowledge of such a heterogeneous ultra-rare syndrome, and, therefore, improve its diagnostic yield, case management, and therapeutic journey for future patients. Keywords: SYT1, hypotonia, congenital malformations, NDD, WES