Abstract: 　Objective　To explore the clinical and gene mutation characteristics of hyperornithinemia-hyperammoniahyperhomocysteinuria (HHH syndrome). Methods　The clinical data of HHH syndrome in a child were retrospectively analyzed and the literature was reviewed. Results　A girl, aged 2 years and 2 months, had an aversion to high-protein foods. The child's reaction was weak after birth, and the ataxia occurred when she walked alone and was aggravated after infection. Laboratory tests showed hyperammonia, elevated transaminase and abnormal coagulation. Head MRI showed decreased white matter volume in the semioval center. The genetic test showed that the child carried compound heterozygous mutations of SLC25A15 gene, c.190T>C (p.Y64H) and c.278G>A (p.R93Q), which were derived from her parents with normal phenotype. Conclusion　The main clinical manifestations of HHH syndrome are involvement of nervous system and liver, and required to be differentiated from other urea circulatory disorders and liver diseases. The newly discovered SLC25A15 gene locus mutation may be the pathogenic site of HHH syndrome.

Key words: 　hyperornithinemia-hyperammonemia-homocitrullinuria syndrome;　HHH syndrome;　hyperammonemia;　 urea cycle disorder;　SLC25A15;　ataxia