Twin studies illustrate the importance of these genetic factors. If the concordance rates in monozygotic and dizygotic twins are about the same, the genetic effect is small. For most autoimmune diseases, concordance rates are 15-30% for monozygotic twins versus 2-5% for dyzigotic twins, consistent with a sizeable genetic effect. Identification of the actual mutations or genetic polymorphisms that confer susceptibility to autoimmune diseases has been complicated by the fact that most autoimmune disorders appear to involve multiple genes, each with only small effect. Moreover, many autoimmune "diseases" are actually "syndromes" that may arise through a variety of different pathogenic mechanisms and genetic abnormalities. Even in the inbred lupus-prone mouse strain NZB/W, ten or more susceptibility loci are thought to contribute to disease severity in an additive fashion (treshold liability model).
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