Bankpeptide routinely synthesizes biotinylated peptides for use in protein-protein interaction studies. Although biotin can be introduced either N- or C-terminally (via lysine residues), we recommend the use of an N-terminal modification owing to its low cost, higher success rate, shorter turnaround time, and ease of operation. Peptides are synthesized from the C-terminus to the N-terminus; therefore, the N-terminal modification is the last step in the SPPS protocol, and no additional specific coupling steps are required. In contrast, a C-terminal modification requires additional steps, and is usually more complex. However, in principle the biotin can be positioned anywhere. Biotin can be separated from the peptide by a variety of different linkers or spacers. Nevertheless, it is recommended that a flexible spacer such as Ahx (a 6-carbon linker) is included to render the biotin label more stable or flexible. Bankpeptide offers biotinylation at either the N- or C-terminus: biotin-N-terminus, Lys-biotin-middle of the peptide, and Lys-biotin-C-terminus. We also offer biotinylation using an Ahx linker or a long carbon (LC) linker: biotin-Ahx-Nterminus, Lys-Ahx-biotin-middle of the peptide, Lys-Ahx-biotin-C-terminus.