Primers are given 5′ to 3′. Primers γCH1 and C-κ were designed as described.15 RFB4 VH5 and RFB4 VL5 were designed according to the N-terminal protein sequences determined by Edman degradation. RFB4 VH5 encodes an Nde I site (bold) and an initiator Met (bold italicized). RFB4 VL3 encodes a HindIII site (italicized). Primers RFB4 VH3 and RFB4 VL3 were designed according to the nucleotide sequence determined from cDNA clones. Primers RFB4 VH3 and RFB4 VL5 include partial Gly₄Ser linker sequences which partially overlap (underlined italicized). Primer RFB4 VL3 dsFv mutates V_L Gly₁₀₀ residue to Cys (underlined), includes a terminator codon (bold) and an EcoRI site (italicized). Primer RFB4 VH dsFv(cys) mutates V_H Arg₄₄ to Cys (underlined). RFB4 VH3 dsFv includes an additional Lys codon and a HindIII site (italicized). RFB4 VL5 dsFv includes an Nde I (bold) and an initiator Met (bold italicized).