ERBB2 and breast neoplasm: Of note, the minimal thresholds of >4 gene copy number is required as replicating cells (G2 phase, in anticipation of cell division) will have four copies of chromosome 17 and HER2 gene, therefore breast tumors with normal HER2 status but high proliferative activity may have a mean HER2 copy number up to 4 (Ross et al., 2003; Szollosi et al., 2005).