Fab-scFv is a type of bispecific fragments. The heavy chain Fd fragment and the light chain of an antibody Fab fragment form natural in vivo heterodimerization. The heterodimerization module can be used as a scaffold, upon which, additional functions, such as variable domains, are incorporated. The Fab and Fv portion can both be against target antigens and serve as dual inhibitors or engagers. This format can also have been applied to extend the half-life of the Fab moiety, with Fab against a target antigen and Fv against serum albumin. The molecule weight of Fab-Fv is ~75 kDa. Base on the structure of Fab-Fv, interdomain disulﬁde bond can be introduced into the Fv domain to generate a Fab-dsFv molecule. Fab-dsFv molecules have also been proven to be biophysically stable and retain afﬁnity for both antigens.