The type of Leigh syndrome found at a much higher rate in the Saguenay-Lac-Saint-Jean region of Quebec is caused by a mutation in the LRPPRC gene, located on the small ('p') arm of chromosome 2. Both compound heterozygosity and homozygous mutations have been observed in French Canadian Leigh syndrome. This subtype of the disease was first described in 1993 in 34 children from the region, all of whom had a severe deficiency in cytochrome c oxidase (COX), the fourth complex in the mitochondrial electron transport chain. Though the subunits of the protein found in affected cells were functional, they were not properly assembled. The deficiency was found to be almost complete in brain and liver tissues and substantial (approximately 50% of normal enzyme activity) in fibroblasts (connective tissue cells) and skeletal muscle. Kidney and heart tissues were found to not have a COX deficiency.