Genetics is the branch of medicine concerned with how hereditary and genetic factors play a role in causing a disease, birth defect, or inherited susceptibility to a health problem such as cancer. Cancers develop due to alterations (mutations) in genes, that when working properly promote normal, controlled cell growth. Only a small percentage of cancers involve inherited mutations that are passed from generation to generation. The majority of cancers can be attributed to acquired mutations. "Acquired" means that the mutations occur only in the tissue that is affected by cancer (for example, colon cancer cells), and are not passed to children. These changes occur at the cellular level after birth, as a result of environmental exposures (such as smoking), lifestyle behaviors (such as eating poorly or not exercising), or chance alone. Mutations in a person's DNA accumulate over time. If mutations affect genes that control cell growth this may cause a cell to grow out of control, and to ultimately become a cancer cell. Therefore, all cancers are genetic, in that they develop because of an accumulation of mutations in genes, but most are not inherited. The percentage of cancers that result from a single inherited factor varies depending on the type of tumor. For the more common cancer types, like breast and colon cancer, less than 10% are inherited. The Human Genome Project began in 1990 with the goal of mapping the location of all of the genes on a cell's chromosomes. This monumental achievement will give scientists the building blocks to determine how diseases such as cancer are caused and hopefully, how to treat them and, ultimately, prevent them.