A syndrome, in medicine and psychology, is the collection of signs and symptoms that are observed in, and characteristic of, a single condition. In medical genetics, a syndrome refers specifically to medical condition where the underlying genetic cause has been identified, and the collection of symptoms is pathogenetically related. Examples of syndromes in medical genetics include: Down syndrome, Stickler syndrome, and Williams syndrome. Conditions such as Toxic shock syndrome and Acquired Immune Deficiency Syndrome are examples of non-genetic syndromes.[citation needed] If the underlying genetic cause is not known, the condition is referred to as an "association". By definition, an association shows that the collection of signs and symptoms occurs in combination more frequently than would be likely by chance alone. Syndromes are often named after the physician or group of physicians that discovered them or initially described the full clinical picture. Recently, there has been a shift towards naming conditions descriptively or according to their underlying cause, however the "eponymous" syndromes often persist in common usage.