The successful conjugation of VHH with small molecules and siRNA led us to challenge even further the bTG, by designing an immunoglobulin G (IgG) containing the "DAIPENLPPL" sequence either at the end of the light chain or at the end of the heavy chain (Table ). The chosen antibody is targeting the transferrin receptor, which is responsible for the cellular iron uptake and is overexpressed in several types of cancer. We tested the conjugation of dansylcadaverine on the unmodified parent IgG_LALA and on the antibody modified on both light chains (IgG_LALA_LC_DAIPENLPPL) or heavy chains (IgG_LALA_HC_DAIPENLPPL). In order to perform a better LCMS analysis of the reaction mixture, the reactions were reduced using urea, ammonium bicarbonate and dithiothreitol (DTT) in order to break the sulfur bridges between the chains, followed by a deglycosylation step using PNGaseF, to prevent the difference of retention time between different isoforms. This treatment resulted in a separation of the light and heavy chains by LCMS (Figures ). Conjugation with dansylcadaverine could be detected on both light chains of the IgG_LALA_LC_DAIPENLPPL with a 12% conversion and a drug to antibody ratio of 2 (Figure and Table ). No conversion could be detected with the negative control IgG_LALA nor on the antibody modified at the end of the heavy chain (Figures and). It is possible that the recognition sequence is more sterically hindered at the C-terminus of the IgG heavy chain than on a VHH domain, which precludes bioconjugation. Structural insight could help to better understand the requirements for activity by bTG. Conjugation of the light chain with siRNA was also achieved in longer reaction time (96 hours) and 10% conversion could be achieved (Figure ).