MKI67 and ductal breast carcinoma in situ: Hence, given that fast-replicating (high Ki-67) tumours might be expected to contain a higher burden of dysfunctional tumour suppressor genes, our study results raise the possibility that pure IDC tumours arise as a result of more drastic tumour suppressor gene defects, whereas IDC-DCIS tumours tend to evolve as a result of a more incremental accumulation of milder suppressor gene defects (Figure 2).