The DPC4 and Madr2 genes are located at 18q21, and the LOH on chromosome 18q21 has been shown to occur frequently in colorectal cancers. To investigate the role of these genes in advanced colorectal cancers, we analyzed 29 colorectal specimens for alterations in the DPC4 and Madr2 genes. Twelve (63.2%) of 19 informative primary colorectal cancers showed allelic loss of chromosome 18q21.3 marker. An alteration of the DPC4 gene sequence was identified in 6 (20.7%) of 29 colorectal carcinomas, and the distinct Madr2 gene mobility shifts were present in 3 (10.3%) cancers. Somatic mutations were identified in these tumors by sequencing analysis. DPC4 gene alterations of 4 cases were detected in Mad homology 2 domains. There was no significant correlation between the somatic alteration of Madr2 and clinicopathological findings. However, the frequency of DPC4 mutation was significantly higher in tumors associated with liver metastasis than in those without such metastasis. Our findings suggest that somatic alteration of the DPC4 gene may play a role in tumorigenesis and liver metastasis of human colorectal cancers.