There are clearly a range of other factors, given that one third of apparently healthy people also have the same genetic polymorphisms.

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Enormous efforts have gone into examining the contribution of other normally-occurring genetic variants (single nucleotide polymorphisms, or SNPs), but these studies have only identified a few SNPS with very slight increases in risk.

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"Thus, E-4 appears as a risk factor for both heart disease and Alzheimer's disease, two major causes of mortality and morbidity at advanced ages, " Francois Schachter and his colleagues at the Center for the Study of Human Polymorphisms, a genetic-research center in Paris, conclude in a report.

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The scientists looked for common genetic variations - known as single nucleotide polymorphisms (Snps) - linked to the three cancers.

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It focused narrowly on areas in and around active genes, ignoring the 98.5% of genomic material known as "junk DNA." This process pinpointed 25, 000 genetic landmarks called SNPS (single nucleotide polymorphisms, or "snips"), places where a single dna letter varies from one person to the next.

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As a first step, the scientists examined half a million genetic markers in the participants' DNA. These markers, known as single nucleotide polymorphisms, or SNPs, can together reveal how genetically similar people are, even though they are not conventionally related.

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