Ten-micrometer sections of formalin-fixed paraffin-embedded blocks of surgically resected, pathologically confirmed cancerous tissue were used to extract DNA just after fixation (QIAamp tissue kit; Qiagen, Courtaboeuf, France). The revised Bethesda Guidelines23 were used to determine MSI status. Analysis of MSI was performed using panel 18 of 5 quasimonomorphic mononucleotide repeat markers (BAT26, BAT25, NR21, NR22, and NR24) that map to intron 15, intron 16, 5′ untranslated region (UTR), 3′ UTR, and 3′ UTR of the MSH2, c-kit, SLC7A8, transmembrane protein precursor B5, and ZNF2 genes, respectively. A single pentaplex polymerase chain reaction (PCR) allows coamplification of all 5 markers, which are subsequently analyzed using a genetic analyzer (ABI PRISM 310; Applied Biosystems, Courtaboeuf, France) and computerized fragment analysis. For each of 5 primer pairs used in the PCR, 1 primer was end labeled with fluorescent dye FAM, HEX, or NED as follows: BAT26 (5′-TGACTACTTTTGACTTCAGCC-3′ and FAM-5′-AACCATTCAACATTTTTAACCC-3′), BAT25 (5′-TCTGCATTTTAACTATGGCTC-3′ and NED-5′-TCGCTTCCAAGAATGTAAGT-3′), NR21 (5′-TAAATGTATGTCTCCCCTGG-3′ and HEX-5′-ATTCCTACTCCGCATTCACA-3′), NR22 (5′-GAGGCTTGTCAAGGACATAA-3′ and FAM-5′-AATTCGGATGCCATCCAGTT-3′), and NR24 (5′-CCATTGCTGAATTTTACCTC-3′ and HEX-5′-ATTGTGCCATTGCATTCCAA-3′). The PCR contained 40 to 50 ng of DNA, 200μM deoxyribonucleotide triphosphate, 2.5mM magnesium chloride, 1μM of each primer, and 1 U of DNA polymerase (AmpliTaq Gold, Applied Biosystems) in a total volume of 20 μL. Cycling variables were 7 minutes at 95°C, 45 seconds at 95°C, and 1 minute at 60°C for 40 cycles, with a final extension step for 10 minutes at 72°C. Two microliters of the completed PCR was added to 20 μL of HiDi formamide (Applied Biosystems) containing 1 μL of ROX (GeneScan 500, Applied Biosystems), vortexed, denatured for 2 minutes at 95°C, and immediately placed on ice. The denatured PCR products were separated by capillary electrophoresis using the genetic analyzer and were further analyzed using commercially available software (GeneScan, Applied Biosystems). Resected specimens were considered MSI tumors if at least 3 markers showed instability.