In order to understand the genetic mechanisms of how genes cause cancer, it is important to review some basic genetic concepts. Genes come in pairs, and work together to make a protein product. One member of the gene pair comes from the mother, while the other member is inherited from the father. Eggs and sperm are called "germ cells." When an alteration or mutation in a gene is present in the germ cells, it is referred to as a "germline mutation." When a germline mutation is inherited, it is present in all body cells. On the other hand, mutations that we are not born with, but that occur by chance over time in cells of the body are said to be "acquired." Acquired mutations are not present in all cells of the body, are not inherited, and are not passed down to our children. Acquired mutations are always involved in causing cancer. Germline mutations are involved in a small percentage of cases.The formation of tumors basically results from cell growth that gets out of control. In the human genome, there are many different types of genes that control cell growth in a very systematic, precise way. When these genes have an error in their DNA code, they may not work properly, and are said to be "altered" or mutated. An accumulation of many mutations in different genes occurring in a specific group of cells over time is required to cause malignancy. The different types of genes, that when mutated, can lead to the development of cancer are described below. Remember, it takes mutations in several of these genes for a person to develop cancer. What specifically causes mutations to occur in these genes is largely unknown. However, mutations can be caused by carcinogens (environmental factors known to increase the risk of cancer). The development of mutations is also a natural part of the aging process.