We have performed molecular genetic analysis of the nm23-H1 gene on 17 Korean colorectal cancer patients. The cDNA, intron I, and intron IV of nm23-H1 gene were used as probes to examine the loss of heterozygosity (LOH) in Korean colorectal tumor. Southern blot analysis using three probes suggested that functional and/or non-functional nm23-H1 homologous gene copies might exist in the human genome. Among the three probes, intron IV probe seemed to be the most specific one under our hybridization condition. Southern blot analysis with colon cancer cell lines showed 7.6 kb and/or 2.3 kb bands. None of the colon cancer cell lines displayed loss of both bands. Southern blot analysis of Korean colorectal tumors and corresponding normal mucosas with Bgill using intron IV probe demonstrated 3 size bands: 7.6, 4.3, and 2.3 kb. This deserves attention because only 7.6 and 2.3 kb bands appeared in Caucasian and Japanese cases. The identity of the 4.3 kb band is not yet understood. Three out of 17 Korean colon cancer patients (17.6%) showed LOH when 7.6 kb and 2.3 kb bands were considered. This frequency is slightly higher than that in Japanese colorectal tumors, where 10.3% of LOH was reported (Okada et ai., 1994). However this frequency is lower than those of two independent studies, in which the LOH of colorectal tumors were 52.3% and 22%, respectively (Cohn et aI., 1991; Leone et aI., 1991a).