The human epidermal growth factor receptor-2 (HER2) gene (also known as c-erbB-2 or neu) is amplified in 20--30% of breast cancers. HER2 gene amplification and HER2 overexpression occur early in the development of breast cancers and are found in a high proportion of ductal carcinomas in situ (DCIS), non-invasive cancers that generally do not give rise to metastases. In DCIS, HER2 overexpression is found specifically in poorly histologically differentiated disease and not in well differentiated cancers. Various methods have been used to analyse the HER2 status of a tumour. These either measure the degree of HER2 gene amplification, receptor overexpression or the amount of circulating HER2 protein. In practice, immunohistochemistry is the most frequently used method, being available as a standard technique in all pathology laboratories. It is of critical importance to standardise the methods used for staining and to apply common interpretation criteria to enable direct comparison of results between laboratories.