1996). High level of HER2 expression has been correlated with poor clinical outcome (Slamon et al. 1987). Several lines of evidence support a direct role for HER2 overexpression in the pathogenesis and poor clinical course of human tumors (Hynes 1993). When the mutated gene is transfected into murine fibroblast (NIH 3T3) cells, it causes transformation, and the resulting cells are tumorigenic in the nude mice (Di Fiore et al. 1987; Hudziak et al.