HST1 (or HSTF1 in human gene nomenclature) is a transforming gene isolated from several cancerous and noncancerous cells. The HST1 protein is a heparin-binding growth factor with significant homology with human fibroblast growth factors and the mouse Int-2 protein. Here, we report the identification of expression of HST1 in a human teratoma cell line and in 5 out of 9 surgically resected human testicular germ cell tumors including seminomas and embryonal carcinomas. Mouse HST1 homologue was expressed in a certain stage of mouse embryo but not in postnatal mice.