= Amino acid residue
An <amino acid residue> is the portion of an <amino acid> incorporated into a <polypeptide chain>. For example, the A in the illustrative <protein primary structure> is an <alanine> residue, rather than a free <alanine> <molecule> floating beside the chain. Counting <amino acid residues> counts chain positions; repeated letters are separate positions. See https://iupac.qmul.ac.uk/AminoAcid/A1416.html[IUPAC-IUB, section 3AA-16: symbols for amino-acid residues].
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