Based on these results we decided to evaluate this sequence for conjugation on other proteins. First, we tested VHH single domain antibodies. VHH antibodies consists in the single variable domain of heavy chain antibodies. It possesses the high affinity and specificity of antibodies and its smaller size grants access to high stability and solubility as well as lower production costs, making them interesting candidates for clinical therapeutics, immunodiagnostics and environmental monitoring. We took as a model a VHH antibody against epidermal growth factor receptor (EGFR), a protein involved in cell proliferation, differentiation, division and survival. EGFR is a well-established oncogenic driver that features high-level expression across multiple cancer indications of interest. EGFR inhibitors could slow down and regulate the EGFR signaling cascade. We first designed seven anti-EGFR VHH constructs: anti-EGFR_VHH consists in the VHH parent sequence as a negative control, anti-EGFR_VHH_ENL possesses the "ENL" conjugating sequence at the C-terminus without linker, anti-EGFR_VHH_ENL_H6 was designed with the "ENL" sequence followed by a His-Tag at the C-terminus, anti-EGFR_VHH_(G4S)3_ENL possesses a flexible linker between the VHH sequence and the conjugation sequence, anti-EGFR_VHH_(AP)4_ENL has a rigid linker between the VHH and the ENL sequences, anti-EGFR_VHH_LPETG_ENL bears the linker previously used in the monobody experiments and anti-EGFR_VHH_DAIPENLPPL is constructed with the longer conjugating sequence right after the VHH sequence. All proteins were successfully produced and analyzed by LCMS (Figures S76 to S82 and Table ). We then performed the conjugation reactions using one equivalent of dansylcadaverine on all systems (Figures S83 to S89, Table and Table ). No conversion was achieved on the negative control composed of the VHH sequence alone, nor on any of the sequences ending by the conjugation tag "ENL". However, 94% conversion was obtained when a His-Tag was placed after the conjugation site and 84% conversion for the longer sequence "DAIPENLPPL". As previous conversion was achieved using the LPETG linker, we believe that the bTG enzyme needs the presence of a few amino acids after the "ENL" conjugation site to be able to perform the conjugation.