Site-directed mutagenesis is a molecular biology technique used to create versions of a gene which contain point mutations – changes to the nucleotide sequence designed to swap out an amino acid in the protein coded by the gene. These changes in the amino acid sequence are often engineered in such a way as to affect some aspect of the protein’s tertiary structure and therefore, its function. For example, a mutant may be created which replaces a serine in the wild type coded protein for an alanine. The side chain of serine contains a hydroxyl (-OH) group which, in the protein may serve as a site where a phosphate group attaches. Alanine has a methyl group (-CH3) to which phosphate cannot attach. The protein created by the mutant gene would therefore not be able to be phosphorylated at this site, which may then affect the activity of this protein in the cell.