A single-chain variable fragment is not actually a fragment of an antibody, but instead is a fusion protein of the variable regions of the heavy and light chains of immunoglobulins, connected with a short linker peptide of ten to about 25 amino acids. The linker is usually rich in glycine for flexibility, as well as serine or threonine for solubility, and can either connect the N-terminus of the VH with the C-terminus of the VL, or vice versa.
This protein retains the specificity of the original immunoglobulin, despite removal of the constant regions and the introduction of the linker. The image to the right shows how this modification usually leaves the specificity unaltered.