Since the human genome has been fully sequenced, and presence of single nucleotide polymorphisms (SNPs) appeared abundant, many studies are associating SNPs with clinical response or even with disease. For some diseases or drug treatments these associations are clear, so that genetic screening for such SNPs or mutations is a standard procedure. For that reason, many different techniques have been developed for fast and easy screening for such specific SNPs/mutations. For reliable screening, the use of controls with known genotypes is indispensable. Plasmids are an ideal tool for making controls which can serve as an inexhaustible source, making new validation superfluous.In this chapter we describe how plasmid controls can be made using DNA with a heterozygous genotype, and also from DNA of which only one allele is available.