scFv(antiHA-12CA5) is an engineered single-chain variable fragment antibody (scFv) that binds the Tag:HA epitope. scFv(antiHA-12CA5) was constructed by combining coding sequence of variable regions of the heavy chain and of the light chain cloned from anti-HA hybridoma clone 12CA5 and consists of nucleotides 1-423 of GenBank LC522514 (heavy chain fragment), a linker sequence (5'-accggtGGC GGAGGCTCTGGCGGAGGAGGTTCCGGCGGAGGTGGAAGCgatatc-3') and nucleotides 67-420 of GenBank LC522515 (light chain fragment) (FBrf0251844).