SNP is the acronym for 'Single Nucleotide Polymorphism' which means that there is a deviation of the normal sequence in a gene segment with a single base mutation. On average, for every 1000 bases (nucleotides) there is one 'spelling' mistake (polymorphism). This means that most people have about 4 to 5 million SNPs in their genome. Most of the time, these spelling mistakes don't matter, but sometimes they do. For example, lactose intolerance in adults is often caused by a SNP that leads to a defective gene for the enzyme that converts the carbohydrate in milk, lactose to glucose and galactose.