A tumor emerges due to the structural and functional abnormalities occurring in the genes, which causes a change in the spectrum of protein molecules. Strong correlations between the gene damages and following changes in the protein spectrum make it possible to study different stages of carcinogenesis and to create a more complete system of molecular markers for the diagnosis of different types of tumors, which comprises protein markers and DNA markers. The present investigation has studied a correlation between the inactivation mechanisms (structural and functional) of the suppressor gene p16/INK4a, which occur at the level of DNA, and the results of its protein expression examined by immunohistochemical methods in the tumor specimens from patients with breast cancer. The investigation could indicate that p16/INK4a gene damage frequently occurred in the tumors of the above type. In the majority of study cases, molecular damages revealed in the gene diminish on its protein expression; however, there are still cases that defy generally accepted explanations.