The limited evidence available suggests that most human cancers are likely the result of genetic transpositions. Through various rearrangements and accumulated mutations, which are now believed to occur commonly in human somatic cells, changes in activation and repression of specific genes, reflected in expression of enzymes, their isozymes and structural proteins, are considered to be the fundamental mechanisms by which the development and progression of a carcinoma are controlled. In this study, I intend to establish systematically the enzyme and isozyme differences and the differences in cell surface proteins between normal and malignant human mammary epithelial cells; the effect of mammotropic hormones on these differential expressions of isozymes and cell surface proteins; and whether this expression is useful in diagnosis and prognosis of human breast cancers. The studies are currently being carried out using a defined system to culture normal and malignant mammary epithelial cells. Work in progress suggest that normal and malignant human mammary epithelium exhibit differences in expression of certain specific enzymes and isozymes. Cell surface reactivities with specific plant lectins have also been found to distinguish normal from malignant breast tumor cells. Efforts are being made to characterize these cellular-biochemical properties unique to neoplastic human breast cells and to identify additional isozymes and cell surface proteins unique in type or amount in malignant human breast tumor cells. The possibility of using these characters as diagnostic and prognostic markers for breast cancer will be assessed.