This book will provide the latest advances in molecular and cellular biology for establishing the foundation of a complete understanding of the mechanisms of breast differentiation leading to cancer prevention. The authors are based on the epidemiological evidence indicating that early first full term pregnancy is a protective factor in human against breast cancer and they have used this paradigm and developed experimental systems in both in vivo and in vitro that have demonstrated mechanistically how the differentiation at the organ and cellular level takes place. This knowledge has provided the blueprint for developing better understanding of the basis of cancer prevention. The transcriptoma analysis of the breast of pre and post-menopausal women has established a genomic signature imprinted in the breast that differs according to the reproductive history of the woman showing that early first full term pregnancy reprogram the organ. This reprogramming takes place at the chromatin level by changing the transcriptional process. The modification of the transcriptional control is due to the expression of non coding RNA sequences and posttranscriptional control driven by the splicesome. The plasticity of the genome of the human breast make possible this reprogramming that is not only induced by the physiological process of pregnancy but by the use of hormones that mimic pregnancy without pregnancy. The author have established the basis of clinical trials for prevention and the discovery that short 15aa peptides of the chorionic gonadotropin hormone can be used in human breast cancer prevention based on preclinical and clinical data.