Domains of PE. The domains of PE include the binding domain (1a); amino acids 1-252; and domains II, Ib, and III, as noted in Figure 1. C3 is a connector (ASGGPE) between VH and the toxin. To allow recombinant immunotoxins to bind selectively to target cells, the VH and VL domains of the mAb are connected to a truncated form of the toxin devoid of its binding domain.