The MCM (minichromosome maintenance protein) protein family was identified for the first time in budding yeast, Saccharomyces cerevisiae. The subgroup consists of MCM proteins 2-9, that possess the characteristic ATPase domain (MCM box). There are also MCM1 and MCM10, which are important in DNA replication, but they do not possess the specific MCM box. The main function of MCM proteins is cooperation with other factors in molecular mechanisms that form the replication fork and in regulation of DNA synthesis. MCM proteins form a ring-shaped complex, which is activated when other factors are bound. MCM 2-7 complex is one of the pre-replication factors. Association of MCM 2-7 complex is a crucial moment initiating the replication fork. MCM proteins play a role in maintaining genome integrity and prevent re-replication once per cell cycle. Proliferating cells have high levels of MCM, whereas they are not detected in quiescent, differentiated or senescent cells. They are also potential useful markers of cell proliferation. Recent studies suggested that MCMs are good markers of proliferation activity degree, because they are highly expressed in a variety of tumors. The aim of this work is to summarize current knowledge about the role of MCM proteins in DNA replication and potential diagnostic markers of proliferating cancer cells.