MLH1 and Leigh syndrome: LS tumors are characterized by genomewide microsatellite instability (MSI; Parsons, Li, & Longley, 1993) which is a marker for germline mutations in the mismatch repair (MMR) genes; MLH1 (Bronner et al., 1994), MSH2 (Leach, Nicolaides, & Papadopoulos, 1993), MSH6 (Miyaki et al., 1997), and PMS2 (Nicolaides, Papadopoulos, & Liu, 1994).