Five oncogenes have been implied as having a role in human breast tumorigenesis: int-2, c-erbB-2 (HER-2), c-myc, c-Ha-ras and the recessive Rb-1. As far as the function and biochemistry of these oncogenes have been studied, they act at different levels and have totally different functions in the cells. They are normally cellular genes, likely to have important functions in normal cell growth or differentiation. In the tumors their regulation or function is altered, due to a wide class of mutations. The oncogenes may cooperate to result in the malignant cell phenotype. However, different oncogenes are mutated in different tumors, so that the tumors show a variable pattern at the molecular level, underlining the individuality of these tumors already described as differences in histopathology, hormone receptor expression and clinical course. The main importance of the oncogene studies is still to reveal basic pathogenetic mechanisms. When appropriate it is important to test diagnostic or prognostic significance of the oncogene mutations.