A
single-chain variable fragment (
scFv) is not actually a fragment of an antibody, but instead is a
fusion protein of the variable regions of the
heavy (V
H) and
light chains (V
L) 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 V
H with the
C-terminus of the V
L, 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.