Polysomy 17 is defined as having more than 2 copies of the chromosome 17 centromere (green) signal. Polysomy 17 is observed in approximately 8% of all specimens, mostly among cases with 4-6 HER2 gene copies. Polysomy 17 should be noted if found. However, the significance of this finding is unknown.
The diagram illustrates polysomy of chromosome 17 (tetraploid in this instance) and would result in 4 copies of HER2/neu and may cause overexpression of protein without actual gene amplification. There would not be an increase in the ratio of HER2/neu to chromosome 17.