Twins needed to join in research
More volunteers are being urged to help double the size of the world's largest existing database for twins.
Doctors from the TwinsUK project need 10,000 more people - 5,000 sets of twins - to help with the study into medical conditions.
The project is being launched to coincide with World Twins Day.
It wants to find clues about health problems such as heart disease, stroke and obesity as well as testing sexual chemistry.
The group has already made genetic breakthroughs in osteoporosis, arthritis, eye disease, acne and back pain.
The researchers also want to look at factors such as sense of humour, musical ability and attraction.
Recruiting centres have been set up in Bristol, Leeds, Manchester, Newcastle, Oxford, Southampton, Stoke, London, Aberdeen and Glasgow.
The £1m-a-year project is initially being funded by biomedical research charity the Wellcome Trust.
Professor Tim Spector, from St Thomas' Hospital in London, said: "The genes of identical twins are the same so comparing them with non-identical twins, who share only half their genes, allows us to separate nature from nurture.
To take part in the study call 0207 188 5555 or email [email protected]
"It also allows us to pinpoint the causative genes and to accurately measure the effects of our environment, such as diet."
Identical and non-identical twins aged 18 upwards are being asked to come forward.
They will have their blood pressure and cholesterol levels checked and may also have a bone density test for osteoporosis as well as cardiac checks.