Non-ketotic hyperglycinemia (NKH) is a rare inborn error of metabolism and is caused by a glycine cleavage system deficiency. Eighty-five percent of patients present with the neonatal type of NKH, the infants initially develop lethargy, seizures, and episodes of apnea, and most often death. Between 60-90% of cases are caused by mutations in the glycine decarboxylase (GLDC). We believed that more mutation reports especially for rare disease as NKH help to evaluate the genotype-phenotype relationship in patients with GLDC. In this study, we describe a case of a neonate admitted to intensive care unit with hypotonia, respiratory failure, lethargy, poor feeding. Due to the history of 2 non-ketotic hyperglycinemia diagnosed male siblings, molecular prenatal diagnosis in patient was performed and a novel c.2963G>A (Arg998Gln) homozygous mutation within the GLDC gene has been detected. We aimed to contribute to mutation knowledge pool of GLDC gene with a novel mutation.