How many amino acids are commonly found in proteins?

There are 20 amino acids that are commonly found in most proteins. These are often referred to as the standard or common amino acids and serve as the essential building blocks for protein synthesis.12

While more than 100 different types of amino acids occur in nature, particularly in plants, these 20 specific types are the ones consistently incorporated into protein molecules. In addition to these standard building blocks, some proteins may contain small amounts of other, less common amino acids, which are often the result of chemical modifications occurring after the protein has been synthesized. Furthermore, selenocysteine is sometimes considered a 21st amino acid because it is incorporated into certain proteins during the process of biosynthesis.234