Final answer:
Hydroxyprolines and phosphoserine, which are not among the 20 proteinogenic amino acids, can still be found in proteins as they are derivatives of the common amino acids and are formed after the protein has been assembled.
Step-by-step explanation:
Proteins are constructed from a set of 20 amino acids known as proteinogenic amino acids. These amino acids are incorporated into proteins during biosynthesis. However, some amino acids like hydroxyprolines and phosphoserine, which are not among the 20 proteinogenic amino acids, can still be found in proteins. This is because these amino acids are derivatives of the common amino acids and are formed after the protein has been assembled.