2. Introduction. 3. Human Breast Epithelial Cells (HBEC) in Culture 4. Factors Influencing Susceptibility of HBEC to Cell Transformation 4.1. Lobular differentiation 4.2. Genetic predisposition 4.3. Cell immortalization 5. Molecular Mechanisms of Cell Immortalization 5.1. Activation of telomerase 5.2. Abrogation of cell cycle control 5.3. Genes preferentially expressed during cell immortalization 6. Molecular Mechanisms of Cell Transformation 6.1. Epigenetic mechanisms 6.2. Genetic mechanisms 7. Genomic changes in Immortalization and Transformation of HBEC 7.1. Genomic changes in cell immortalization 7.2. Genomic changes in cell transformation 8. Genomic Changes in Human Breast Lesions 9. Functional Roles of Chromosomes 11 and 17 in Transformed Phenotype Expression of HBEC 10. Summary and Perspectives 11. Acknowledgments 12. References