Which of the Following Is True About Uncommon Amino Acids

Multiple Choice

Which of the following is true about uncommon amino acids?

A) They are found in all proteins
B) When they are found in proteins, they are first created by modification of the parent amino acid and then incorporated into the protein
C) They are formed in the protein by post-translational modification of the parent amino acid
D) They are always based on modification of tyrosine
E) none of the choices