In vivo production of mAb by additive gene therapy. The mAbs can be produced by direct or indirect injection (using viruses such as lentiviruses or AAVs) of DNA encoding the desired antibody. They can also be produced by the injection of mRNA carried by lipid nanoparticles. mAbs are then detected in biological fluids. It is possible to obtain different antibody formats such as a complete immunoglobulin, an ScFv, or a nanobody depending on the original sequence.