How many standard amino acids are used to build most proteins?

There are 20 standard, or common, amino acids that are considered the essential building blocks of all proteins.1

While these 20 amino acids are the primary components used in protein biosynthesis, it is worth noting that selenocysteine is sometimes considered a 21st amino acid because it is incorporated directly during the translation process rather than being modified after the protein is formed. Additionally, many proteins contain nonstandard amino acids that result from chemical modifications occurring after the protein has been synthesized.2