Mutations at the nucleotide positions 3243, 3252, 3256, 3271, and 3291 of the mitochondrial tRNALeu gene have been found in some patients with MELAS (mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes) and sensorineural hearing loss (SNHL). Among these syndromic SNHL-causing mutations, M.3243A>G and M.3271T>C are the two most common mutations found in patients with MELAS or MELAS with deafness. This test does not rule out the possibility of low-level heteroplasmy for these mutations or some other mitochondrial mutations. This laboratory performs targeted mutation analysis.