A phenotype is one or more observable features or traits that report changes in a biological system or its reaction its environment. Phenotypic screening is the systematic identification of agents (such as small molecules, biological molecules or genetic mutations) that alter a phenotype.
Phenotypic changes are a factor in most diseases, whether it is a cancer cell undergoing uncontrolled cell division, a motor neuron that fails to connect to muscle tissue, or the complex defects seen in the brain of a patient with schizophrenia.