An organism's complete set of DNA is called its genome. Normally every single cell in the human body contains a complete copy of the approximately 3 billion DNA base pairs, or letters, that make up the human genome. Genomics includes the scientific study of complex diseases such as heart disease, asthma, diabetes, and cancer because these diseases are generally caused more by a combination of genetic and environmental factors than by the individual genes. Genomics is offering new possibilities for therapies and treatments for some complex diseases and also new diagnostic methods.