How are tumors formed?

Tumors are masses of cancer cells that pile up. It starts with one mutation, turning a regular cell into a cancerous cell. The cancerous cell behaves abnormally in that they have no signal to tell them when to start and stop reproducing so they are continually doing so. Oncogenes turn normal cells into cancer cells by interfering with these dividing signals. The suppressors will try to stop cancer cells from behaving abnormally, but the cancer cells usually overcome the suppressors. The cells can also destroy themselves in a process called apoptosis, but once the cells become cancerous, then apoptosis is not possible. Scientists have developed several drugs to treat cancer but the cells that escape treatment metastasize and form new, stronger, drug-resistant tumors.