If you are a non-identical twin the risk falls to around 10 percent in most studies.

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The festival has no parallel in southern Europe, but it's Christmas's non-identical twin: it holds a special place in the national psyche with its own customs, memories and social pressure.

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The Twin Research Unit is looking to extend their work to identify the exact genes responsible in this and other disease areas and would like to hear from Twin Volunteers aged over 18, either identical or particularly non-identical who can call the twin hotline on 0990-770099.

BBC: Identical twins

If you are a twin (identical or non-identical) aged over 15 who would like to volunteer to join the 10, 000 twins on the register of the Twin Research Unit at St Thomas' Hospital, call 020 7188 5555.

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