DNAs from 21 human stomach cancers, 16 metastatic stomach cancers to lymph nodes, and 21 non-cancerous mucosae of the stomach from a total number of 26 patients with stomach cancer were assayed for their transforming activity to NIH3T3 cells. Three samples of DNA were positive in transfection assay; one was from a primary stomach cancer of one patient (No. 361), the second was from a non-cancerous portion of stomach mucosa of the same patient (No. 363) and the third was from a lymph node metastasis of stomach cancer from another patient (No. 51). A portion of the transforming gene was cloned and a cDNA clone for this gene was isolated. The reading frame essential for the transforming activity was identified. From the results of sequence analysis of cDNA and Southern blot analysis, this transforming gene did not have homology with any oncogene reported previously. We applied the term, hst (human stomach cancer), to this novel transforming gene. This hst gene was not only responsible for acquisition of transforming activity of No. 361 DNA, but also for that of No. 363 and No. 51 DNAs; all the transformants induced by Nos. 361, 363, and 51 DNAs contained hst gene.