The invention relates to a method of making a polypeptide comprising an orthogonal functional group, said orthogonal functional group being comprised by an aliphatic amino acid or amino acid derivative, said method comprising providing a host cell; providing a nucleic acid encoding the polypeptide of interest; providing a tRNA-tRNA synthetase pair orthogonal to said host cell; adding an amino acid or amino acid derivative comprising the orthogonal functional group of interest, wherein said amino acid or amino acid derivative is a substrate for said orthogonal tRNA synthetase, wherein said amino acid or amino acid derivative has an aliphatic carbon backbone; and incubating to allow incorporation of said amino acid or amino acid derivative into the polypeptide of interest via the orthogonal tRNA-tRNA synthetase pair. The invention also relates to certain amino acids, and to polypeptides comprising same.