Medically called malignant neoplasm, cancer is the general, widely-used term for a group of different diseases characterized by abnormal cells that divide and grow uncontrollably, forming malignant tumors. Being the second leading cause of death, cancer is alarmingly emerging as a major public health problem in the United States and the world. In 2013, cancer accounted for an estimate of one out of four deaths in the United States, taking away approximately 580,350 lives [15]. Moreover, the American Society of Clinical Oncology has recently predicted that cancer would eventually surpass heart diseases to become the leading cause of death in the United States in 16 years [18]. In addition to the human cost of lives, cancer also has serious social and economic implications. As estimated by the National Institutes of Health, the total annual cost of cancer in 2009 was $216.6 billion, of which $86.6 billion was direct medical cost and $130 billion was indirect cost of loss productivity due to premature death caused by cancer [8]. A lot of researches have been done on cancer; and yet, our understanding of cancer is still very limited. In this paper, we will examine the complexity of cancer, specifically focusing on breast cancer “ the second most common cancer among American women [2]. We will study the causes of cancer and its progression from cell, to tissue, to organ then to organ systems; thus, giving us a good understanding of how everything in our body is connected and how cancer, a disease that originates in cell, can affect the whole body. Cancer is a disease of the cell, the most basic unit of life in the body. All human life begins with one cell that constantly reproduces and divides into millions of cells that are grouped together to form human tissues and organs. Through an important process of cell division where a single cell reproduces into two daughter cells, our body constantly produces more and more new cells as they are needed to keep us healthy, replacing dead cells and allowing our body to grow. The process of cell division and growth is controlled by genes that are located in the cell nucleus and contain coded messages for making different types of proteins. Functioning as molecular on/off switches, these proteins control cells, telling them how to grow, work, reproduce and die in an orderly and systematically fashion [16]. This genetically programmed life cycle of cells allow us to grow, repair injuries and keep us healthy. Unfortunately, genes can become mutated as a result of permanent changes in the DNA sequence that makes up a gene. Most mutations are automatically repaired by the cell's own DNA repair machinery; but in rare cases, mutations don't get repaired but are passed on to the offspring cells. Eventually, several genetic mutations that accumulate over time will create abnormal cells that grow and divide uncontrollably, leading to formation of malignant tumor that causes cancer [13]. Moreover, there are over 100 types of cancer and the type of cancer is generally determined by the location of the first abnormal cell. Particularly, breast cancer gets its name because it originates from a single abnormal breast cells. Generally speaking, genetic mutations can occur in two ways. The first one is germline mutations which are gene mutations that are inherited from parent(s). Because these mutations are present in the egg or sperm cells of the parents, they will be inherited and present in every cell in the offspring's body [13]. Cancer that is caused by germline mutation is generally called hereditary cancer. In the case of breast cancer, germline mutation predisposes affected individual to the risk of hereditary breast cancer, accounted for approximately 10% of all breast cancers. Specifically, the germline mutations in BRCA1 and BRCA2 tumor suppressor genes have been identified as the main contributors to hereditary breast cancer, accounting for about 20 to 25 percents of all cases. BRCA1 and BRCA2 germline mutations also tend to cause affected individual to develop breast cancer at younger ages than sporadic breast cancer “ cancer that occurs randomly and is not inherited [12].