Some 20 different kinds of amino acids are found in proteins.

Hydroxyproline is found in few proteins other than collagen.

Related regions are found also in archaebacterial and eukaryotic proteins.

However, only 20 different amino acids are commonly found in proteins.

Palindromes also appear to be found frequently in proteins, but their role in the protein function is not clearly known.

There are only 20 amino acids found in proteins.

It is not found in natural proteins.

A 3 helix is a type of secondary structure found (rarely) in proteins.

Additionally, it is also found in several hypothetical proteins.

Many additional terms are used for various kinds of structural features found in proteins.