The results of research program "Tumor Biology" have significant impact to the broader knowledge about two basic biological processes in cell, that are cell division and apoptosis.Our investigations especially focus on research of both biological processes in malignant alteration cell. Uncontrolled balance between cell proliferation and apoptosis is the basic characteristic of malignant cell. Our interest was particularly molecular mechanism that control the proliferation and apoptosis, esspecially those that can acts as molecular target in cancer treatment. The results of our investigations contibute the significant part to the basic knowledge of appearance and development of tumor cell and the way the tumor cells create proper environment for themselves. Knowledge the specific characteristics and diferences between normal and tumor cell gives the opportunity for new treatment of neoplastic lesions. All kinds of cancer we investigated, laryngeal, prostatic, liver and thyroid gland cancers, are the most frequent kinds in the world. We point at the importance of proliferative and apoptotic activity of tumor cells for the cancer tratment and prediction of cancer prognosis. Model of cancerogenesis "in vitro" conditions, using FRTL-5 cells, will serve for further investigations of cell alterations, when the cells are in the begining of transformation, in the form of precancerosis. We are the first who describe the malignant alteration of standardized thyroid cell line after long cultivation in "in vitro" conditions.