tRNA-guanine transglycosylase (TGT) is an enzyme which synthesizes a modified nucleoside, queuosine, by exchanging the base moiety of guanosine for queuine in tRNA. We have reported that the expression level of the 60-kDa subunit of TGT (TGT60kD) is elevated in leukemic cells, however, there is no other report on the expression of TGT60kD in cancer cells. The expression levels of the TGT60kD protein are elevated in four of the five colon cancer cell lines and 83% of colon cancer tissues compared with normal tissues. The expression levels of the TGT60kD protein decreased in two colon cancer cell lines, after cell differentiation was induced. A marked positive staining of cancer cells in colon tissues was observed, and the subcellular staining pattern was mainly cytosolic. These data suggest that the role of TGT60kD in colon carcinogenesis.