Methylation of promoter regions and frameshift mutations in microsatellites of the coding sequence (CDS) of genes are frequently associated with loss of expression in microsatellite instable (MSI) colorectal carcinoma. In a panel of 40 MSI and 24 microsatellite stable (MSS) colorectal tumours as well as six cultured colorectal carcinoma cell lines hypermethylation of the TIMP3-promoter was found in 28% of MSI and 25% of MSS tumours, respectively. Additionally, three MSI tumours and one cell line displayed instability of a C7-repeat located in the CDS of the TIMP-3 gene. TIMP-3 fulfils all important criteria for being a target gene in the mutator pathway. Thus, TIMP-3 might be a factor of general importance for colorectal carcinogenesis.