Microarrays will also be useful in studying drug metabolism. Drugs are broken down in the body by proteins, and not all of these proteins are identical from person to person. Genetic variations, called single nucleotide polymorphisms (SNPs), can result in subtle differences in proteins that translate to major differences in how the protein functions. For example, as many as 40 percent of people have a SNP that causes a deficiency in a protein called cytochrome P450 CYP2C9, which is important to drug metabolism. Deficiencies in this protein can impair the metabolism of as many as 1 in 5 medications currently on the market.