As with EGFR, increased expression of HER3 is seen in several cancer forms[18] and although it has a severely impaired intrinsic tyrosine kinase activity [11, 13, 17], making heterodimerization with other HER family members essential[12, 42], HER3 has been demonstrated to function as a, possibly HER2-dependent, oncogene[11, 23, 43]. This evidence concerns the gene EGFR and cancer.