The drugs known as targeted therapy help stop cancer from growing and spreading. They work by targeting specific genes or proteins. These genes and proteins are found in cancer cells or in cells related to cancer growth, like blood vessel cells. Doctors often use targeted therapy with chemotherapy and other treatments. Knowing how cancer cells develop helps understand how targeted therapy works. Cells make up every tissue in your body. There are many different cell types, such as blood cells, brain cells, and skin cells. Each type has a specific function. Cancer begins when specific genes in healthy cells change. Scientists call the change a mutation. Genes tell cells how to make proteins that keep the cell working. If the genes change, these proteins change, too. This makes cells divide abnormally or live too long. When this happens, the cells grow out of control. The out-of-control cells form a tumor.