There are many examples of differences between ethnically defined populations with regard to drug-metabolizing enzymes i.e. enzymes that serve as defences against exogenous chemicals. The prevalence of variants of enzymes such as monooxygenases, dehydrogenases, esterases and some transferases is not uniform, and this leads to differences in metabolism of drugs such as aspirin and diazepam, among others. Werner Kalow explains why the mere counting of a genetic variant may not be sufficient to uncover more major problems such as may be posed by multiple allelism or heterozygosity, and why it may be predicted that a large number of drugs will display interethnic differences in function.