ERBB2 and ductal breast carcinoma in situ: Another hypothesis is that HER2 over-expression in invasive carcinoma does not develop from DCIS but from the associated atypical hyperplasia.62,63 Another theory explaining this phenomenon is that most invasive cancers develop from DCIS tumours, which have low expression of HER2 but are highly proliferative.54 Because such tumours progress rapidly, they are in the DCIS stage for only a short period and would therefore be under-represented in population samples.54