Type 1 is the most common form of primary hyperoxaluria, also called oxalosis when systemic involvement has occurred. This recessive autosomal inherited inborn error of metabolism is characterized by a defect of alanine: glyoxylate aminotransferase (AGT), which is a specific liver enzyme. This protein is responsible for glyoxylate detoxification only when it is located in the peroxisome. The clinical and biochemical phenotypes are neither correlated with the residual catalytic activity of AGT nor with its immunoreactivity. Most patients display less than 2% catalytic activity (enz-) or no immunoreactive protein (crm-); peroxisome-to-mitochondrion mistargeting is the main feature of patients crm+/enz+ or crm+/enz-. The cDNA and genomic DNA have been cloned and sequenced and the gene has been located on the long arm of chromosome 2 in the q36-37 region. Three polymorphisms have been identified which are preferentially associated, leading to two alleles; six point mutations have been currently reported.