hst is a transforming gene first identified from transformed NIH 3T3 cells that were transfected with DNA of a human stomach cancer. A genomic fragment of hst obtained directly from a human genomic library also has transforming activity. This fragment has a coding sequence identical to that of the hst cDNA prepared from an NIH 3T3 transformant induced by DNA from a stomach cancer. The deduced amino acid sequence of the hst protein is 43%, 38%, and 40% homologous, respectively, to human basic fibroblast growth factor, human acidic fibroblast growth factor, and mouse int-2 protein in selected regions. This suggests that hst encodes a protein related to fibroblast growth factors, which are wide-spectrum mitogens, and to the int-2 protein, a potential oncogene product implicated in murine mammary carcinogenesis.