Despite not being encoded by the genetic code as proteinogenic amino acids, some non-standard amino acids are nevertheless found in proteins. These are formed by post-translational modification of the side chains of standard amino acids present in the target protein. These modifications are often essential for the function or regulation of a protein; for example, in Gamma-carboxyglutamate the carboxylation of glutamate allows for better binding of calcium cations,[15] and in hydroxyproline the hydroxylation of proline is critical for maintaining connective tissues.[16] Another example is the formation of hypusine in the translation initiation factor EIF5A, through modification of a lysine residue.[17] Such modifications can also determine the localization of the protein, e.g., the addition of long hydrophobic groups can cause a protein to bind to a phospholipid membrane.[18]