The human body consists of 100 trillion cells divided into several organ systems and tissue types. These specialized tissues consists of groups of similar cells, e.g. liver cells, skin cells, nerve cells etc. Each cell contains its own copy of the human genome, the chromosomes that carry the instructions for how to make a complete human being. In the human genome there are 25,790 confirmed genes . In differentiated, mature tissues, most chromosomes are turned off, except for a few genes that are necessary for housekeeping and essential cell function.
Genes are the instructions to make a given protein.
Proteins can be enzymes, which enable chemical reactions, or they can be structural materials such as tubulin which give cells form and shape or collagen which connects them to each other. In breast cancer, cells in the milk-producing lobules or ducts that lead to the nipple grow rapidly in an uncontrolled manner forming a mass of unwanted tissue called a tumor. Cancer cells express proteins that dissolve collagen and enable them to break away and travel to other parts of the body. These proteins are called serine proteases, or MMP's. They are similar to digestive enzymes.