An unusual amino acid used in Solid Phase Peptide Synthesis (SPPS) for the synthesis of photoreactive affinity labels.1-3 The side-chain azido (N3) group is stable in trifluoroacetic acid (TFA) or piperidine (Pip), two reagents commonly used in SPPS. The azido group can be readily converted to an amine on the solid phase or in solution via the Staudinger reduction.4