Despite of long history of the immunotoxin development, none was commercialized yet.It might be mainly due to solubility of immunotoxins composed of 4 chains (about 150 kDa) and a unit of toxin.Because they are composed of multiple protein chains, they aggregate after conjugating antibody with toxin genetically or chemically, preventing mass production.Single chain antibodies (ScFv) or Fab fragments were developed to enhance tissue penetration and molecular solubility.