The MSI positive status of a tumor was scored when additional bands (peaks) were present in certain alleles of the malignant tissue in comparison with the normal counterpart. Identical results for all markers were obtained by PCR-PAGE analysis and fluorescent PCR/capilary electro­phoresis (Fig. 1). MSI was present in 14 out of 107 (13.1%) tumors, 11 of which (10.3%) were classified as MSI-H (instability in three or more markers) and three as MSI-L (instability in only one marker). The MSI phenotypes for each of the seven markers used in the MSI positive tumors are given in Table 1. BAT26 exhibited MSI in all MSI-H tumors and in none of the MSI-L tumors. The frequency of MSI at the D17S250, D18S61, D18S58 and DCC in the MSI-H tumors was 81.8, 72.7, 63.6 and 45.5%, respectively. The three tumors with the MSI-L phenotype showed instability in only one marker each, i.e., at the D18S58, D18S61 and DCC loci. These tumors did not show alterations in the two additional markers used, i.e., at the D2S123 and D5S346 loci. The frequency of MSI at all loci in MSI-H and MSI-L tumors is shown in Table 2.

The association of MSI status with several clinical and histopathological features in tumors of 107 colorectal cancer patients included in this study is shown in Table 3. Statistically significant correlation was found between MSI (MSI-L tumors were considered as MSS) and proximal localization (p <0.0001), nodal expansion (p = 0.008) and Dukes' early stages of the tumors (p = 0.02). No correlation was found between MSI status and either age, gender of the patients, size (T) and histopathological grade (G) (p >0.05 in all instances), while the association of the MSI status with the mucinous histotype was with a borderline significance (p = 0.056).