SNP (Single Nucleotide Polymorphism) - a DNA sequence marker, formed by one nucleotide substitution (e.g. A→ C) at a specific point. It represents a variation in the genetic code which may be different in different people. These small variations, however, may be very numerous for there are 30 million SNPs in a human genome. This very variability dictates the phenotype differences among individual people. Research in this area was fuelled by the finding that certain SNPs are linked with or more prevalent in certain diseases or traits. The results of these studies (association studies) form a theoretical background for the services that GenePlanet provides.