The Mendel Institute in Rome houses a Twin Register that was initiated by the senior author even before the opening of the Institute (September 1953).

The purpose of the Twin Register, originally limited to research on twin concordances as to blood glutathione levels (a subject that is still under investigation, as shown by the report presented at this Symposium by Gedda and Tatarelli), was gradually widened to cover the full horizon of twin studies.

The realization that the twin method, in its various formulations, is irreplaceable in most areas of biomedical research, led to the decision to include in the Register each and every contactable twin pair; wide use of mass media was made for recruitment purposes.

Recruitment generally occurred at very early ages, and all pairs are constantly followed. Formerly, definite zygosity determinations were obtained only for those pairs that were selected for specific research purposes. Zygosity determinations are currently being extended to all those pairs who come to the Institute's clinics, within the limits of the available budget (it should be noted that the Institute is a private foundation which provides free medical care to any twin. Lack of capital funds forces the Institute to limit its research activities to those for which grants are obtained).