Abstract: An expression library comprising a repertoire of nucleic acid sequences cloned from a non-immunized source, each nucleic acid sequence encoding at least part of a variable domain of a heavy chain derived from an immunoglobulin naturally devoid of light chains (VHH) wherein the extent of sequence variability in said library is enhanced compared to the corresponding naive expression library by introducing mutations in one or more of the complementarity determining regions (CDRs) of said nucleic acid sequences or by generating alternative combinations of CDR and framework sequences not naturally present in the naive library repertoire. Also disclosed is the use of such a library in preparing antibodies, more particularly antibody fragments.