 The way a person responds to a drug this includes both positive and negative reactions is a complex trait that is influenced by many different genes. Without knowing all of the genes involved in drug response, scientists have found it difficult to develop genetic tests that could predict a persons response to a particular drug. Once scientists discovered that peoples genes show small variations or changes in their nucleotide DNA base content, all of that changedgenetic testing for predicting drug response is now possible. Pharmacogenomics is a science that examines the inherited variations in genes that dictate drug response and explores the ways these variations can be used to predict whether a patient will have a good response to a drug, a bad response to a drug, or no response at all. 