How many types of amino acid are commonly used to build proteins?

1. Proteins are polymers of amino acids arranged in a linear sequence. 2. The same small alphabet of amino acids builds every protein. 3. All proteins are made from different arrangements of the 20 most common amino acids. 4. Their order, not their number, is what makes each protein different. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 3 'Biological Macromolecules', sections 3.1-3.5_