Do we need more twin studies? The Healthy Twin Study, Korea.

The success of the Human Genome Project and recent technological progress have made the analysis of individual genetic variation much more feasible. However, genetic variations responsible for common complex human diseases and traits are largely unknown, with a few exceptions of specific subtype of common diseases (e.g. early onset type diabetes or Alzheimer disease). 1 Twin study has evolved from the classical twin study design, in which comparison of concordance in traits or diseases between monozygotic and dizygotic twins provided evidence about the genetic and environmental contributions to the phenotypes/diseases of interest. 2 Current twin studies in Europe, Australia, and other countries are already comprehensive genomic studies rather than a classical twin design, which maximize the presence of twins. Examples of innovation include: linkage study using dizygotic twin pairs; linkage and association study using the family of twins; epigenetic study using monozygotic twins; and tests for gene–environmental interaction using twins. The list is ever growing as creative minds apply themselves. 3,4 Although contemporary twin study has the potential to incorporate multidimensional genetic approach, most existing twin studies were started in the 1970s and 1980s, when genomic information and families of twins were rarely collected. It was not until after the 1990s that the availability of genomic information and the value of recruiting families of twins started to be emphasized. Furthermore, most twin studies have been performed in Caucasian populations, which have different genetic variations as well as different cultures and environments from the other twin collections. Twin studies in low-income and middle-income countries