Molecular biology studies have led to the identification of two different types of colorectal carcinomas. The first group, called LOH (for loss of heterozygosity), represents 80% of colorectal cancers and is characterised by aneuploidy, allelic losses and a location in the distal colon. The second group displays phenotypic microsatellite instability (MSI-positive tumours), has a near-diploid karyotype and a relatively low frequency of allelic losses. It accounts for 15% of all colorectal cancers and for about 30% of right-sided cancers. Four different pathways have been identified as responsible for tumour progression: the WNT/Wingless, the K-ras, the Transforming growth factor (TGF) and the P53 pathways. The involvement of these pathways depends on the tumour type. In LOH-positive tumours, the WNT/Wingless pathway is activated through an APC mutation, whereas MSI+ tumours do so through a catenin stabilising mutation. The TGFb growth inhibitory pathway is altered either by mutations in the signal transduction molecules SMAD2 and SMAD4 in LOH positive tumours or by mutations of TGFbRII in MSI+ tumours. In the p53 pathway, mutations in BAX may contribute to the adenoma-carcinoma transition just as p53 mutations may do in LOH positive tumours. Until now, cancer phenotype determination has had no clinical implications. However, the predictive value of the MSI status was recently stressed as a predictive factor for response to chemotherapy. Immunohistochemistry could represent a complementary strategy to molecular biology in assessing MSI status. This simple test would allow to screen all colorectal carcinomas for MSI status, which would provide valuable management information in addition to the histological assessment for tumour stage and grade.