The present invention relates to a method for generating or cloning a nucleic acid or nucleotide sequence that encodes a heavy chain antibody or an antigen-binding fragment thereof, wherein said heavy chain antibody or antigen-binding fragment is directed against a specific antigen, said method comprising the steps of providing a sample or population of cells from a Camelid immunized with said antigen, isolating from said sample or population said at least one cell that expresses or is capable of expressing a heavy chain antibody directed against said antigen, and obtaining from said at least one cell a nucleic acid or nucleotide sequence that encodes a heavy chain antibody directed against antigen or that encodes an antigen-binding fragment thereof directed against said antigen.