The present invention provides a mammalian cell transfected with an expression vector comprising a human protooncogene which has the nucleotide sequence of SEQ ID NO: 1; a mammalian embryo carrying a nucleic acid construct comprising the human protooncogene; a transgenic cancer mammal, which is derived from the mammalian embryo; and a kit for diagnosing a cancer selected from the group consisting of breast, kidney, ovary and stomach cancers, which comprises a probe having the nucleotide sequence complementary to mRNA transcribed from the human protooncogene or a portion of the mRNA; or an antibody binding specifically to a protein translated from the mRNA or a portion of the protein.