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Breast cancer is predominantly caused by unrestrained cell proliferation. Proliferation is a complex process mediated by a network of signals that converge to a point called the 'initiation of genome replication' after which either proliferation or cell death could take place. The minichromosome maintenance (MCM) proteins are located at this point and play a pivotal role in regulating DNA replication. The detection of an aberrant level of such proteins can be of use in early breast cancer diagnosis. The main aim of this thesis was to propose a new system to detect inappropriate cell proliferation in breast epithelium. An in vitro model using cancer cell lines was developed to lay the foundation for subsequent studies employing human breast specimens. The application of the in vitro findings in breast excisions allowed assessing of the specificity and sensitivity of the biomarkers to ascertain slowly proliferating neoplastic cells. The most striking finding of this study was the abnormal presence of the MCM proteins in tumour compared to normal tissues with a typical pattern of expression unique for the histological classification of the lesion. The potential of MCM proteins as indicators of cell proliferation defects was further investigated with association studies with Ki-67, Bcl-2 and ER. MCM consistently identified a higher proportion of proliferating cells compared to Ki-67 suggesting that they are interesting markers of the Gi/S-phase. In fact, the MCM proteins start to co-localise in early Gi whereas Ki-67 is almost absent in this phase. Importantly, MCM proteins could recognise not only the proliferating compartment of the tumour but also those cells with replication potential. Based on these findings, the novel MCM biomarkers can be helpful in identifying both malignant and potentially malignant breast tissues. This feature can be useful in predicting patients at risk of tumour progression.