ERBB2 and neoplasm: However, HER2 status should be determined conversely depending on the IHC findings for the additional three subgroups of cases, which include HER2/CEP17 ratio ≥2.0 and an average HER2 gene copy number <4.0 per tumor cell (group 2), HER2/CEP17 ratio <2.0 and an average HER2 gene copy number ≥6.0 per tumor cell (group 3), and HER2/CEP17 ratio <2.0 and an average HER2 gene copy number >4.0 and <6.0 per tumor cell (group 4).