Library construction. (A) The segments containing VHH gene fragments were amplified by a first PCR. (B) The fragments were amplified by a second, nested PCR. (C) Size of the library was determined by counting the number of clones after serial dilutions and plating on plates containing selective antibiotics. (D) Clones were randomly selected to detect the percentage of clones with a phagemid containing an insert of a proper size for a VHH.