The present invention provides paramyxoviral vectors expressing polypeptides that comprise antibody variable regions. A vector of this invention, encoding antibody variable regions of the H and L chains, succeeded in simultaneously expressing these antibody chains to form a Fab, and further succeeded in expressing a single chain antibody at a high level. The vectors of this invention are suitable as vectors for gene therapy, to be administered in vivo or ex vivo to living bodies. In particular, vectors expressing antibody fragments against neurite outgrowth inhibitors are useful in gene therapies for nerve lesions. Further, vectors of this invention that express antibodies which inhibit immune activation signal transduction enable the long-term expression of genes from the vectors.