Twin research is a genetic investigation method that allows for the determination of whether and to what extent a disease is influenced by hereditary factors in humans. Twins are typically classified into identical (monozygotic, MZ) and same-sex fraternal twins (dizygotic, DZ), as well as opposite-sex pairs of twins. MZ twins are genetically identical individuals, while DZ twins differ in some of their genetic traits, much like siblings. The presence of certain traits or diseases in only one of the twins is generally not hereditary and can thus be attributed to environmental factors. Siemens formulated the "twin pathological inheritance rule" as follows: "All hereditary and all hereditary disposition-related ailments are encountered less frequently together in DZ than in MZ." This is primarily the achievement of the genetic researcher and dermatologist.