Twin Research and Human Genetics


The USC Adult Twin Cohorts: International Twin Study and California Twin Program

Wendy Cozena1a2a3 c1, Amie E. Hwanga1, Myles G. Cockburna1a2, Ann S. Hamiltona1a2, John Zadnicka1 and Thomas M. Macka1a2a3

a1 Department of Preventive Medicine, USC Keck School of Medicine, University of Southern California, Los Angeles, CA, USA

a2 Norris Comprehensive Cancer Center, USC Keck School of Medicine, University of Southern California, Los Angeles, CA, USA

a3 Department of Pathology, USC Keck School of Medicine, University of Southern California, Los Angeles, CA, USA


The study of twin subjects permits the documentation of crude heritability and may promote the identification of specific causal alleles. We believe that at the current time, the chief research advantage of twins as subjects, especially monozygotic twins, is that the commonality of their genetic and cultural identity simplifies the interpretation of biological associations. In order to study genetic and environmental determinants of cancer and chronic diseases, we developed two twin registries, maintained at the University of Southern California: The International Twin Study (ITS) and the California Twin Program (CTP). The ITS is a volunteer registry of twins with cancer and chronic disease consisting of 17,245 twin pairs affected by cancer and chronic disease, respectively, ascertained by advertising in periodicals from 1980–1991. The CTP is a population-based registry of California-born twin pairs ascertained by linking the California birth records to the State Department of Motor Vehicles. Over 51,000 individual California twins representing 36,965 pairs completed and returned 16-page questionnaires. Cancer diagnoses in the California twins are updated by regular linkage to the California Cancer Registry. Over 5,000 cancer patients are represented in the CTP. Twins from both registries have participated extensively in studies of breast cancer, melanoma, lymphoma, multiple sclerosis, systemic lupus erythematosus, diabetes mellitus type 1, mammographic density, smoking, and other traits and conditions.

(Received October 22 2012)

(Accepted October 25 2012)

(Online publication December 07 2012)


  • international twin study;
  • California Twin Program;
  • cancer;
  • chronic disease;
  • etiology;
  • twins


c1 Address for correspondence: Wendy Cozen, USC Keck School of Medicine and Norris Comprehensive Cancer Center, University of Southern California, MC-9175, Los Angeles, CA 90089-9175, USA. E-mail: