Here's a breakdown:

* Proteins are large, complex molecules made up of long chains of smaller units called amino acids.

* Amino acids are the building blocks of proteins. There are 20 different common amino acids that are used to build proteins in the human body.

* Polypeptides are chains of amino acids linked together by peptide bonds. Proteins are made up of one or more polypeptide chains.

So, while the term "subunit" can be used to refer to individual polypeptide chains within a larger protein, the fundamental building blocks of proteins are amino acids.