Given a scientific passage and an answer, generate a question for the given answer.

Passage: A protein is an organic compound made up of small molecules called amino acids . There are 20 different amino acids commonly found in the proteins of living organisms. Small proteins may contain just a few hundred amino acids, whereas large proteins may contain thousands of amino acids. The largest known proteins are titins, found in muscle, which are composed from over 27,000 amino acids. Answer: titins
What are the largest known proteins called?