Our bodies are formed from between 50 and 100 trillion cells (a trillion is a thousand billion, or a thousand, thousand million). These cells are organized into tissues, such as skin, muscle, and bone. Each cell contains all of the organism's genetic instructions stored as DNA. However, each cell uses only the instructions from part of the DNA. For example, a muscle cell uses the DNA that specifies the muscle apparatus, whereas a nerve cell uses DNA that specifies the nervous system.