PromoFluor Maleimides are intended for covalently labeling biomolecules containing a thiol group (e.g. proteins). A neutral thiol-reactive label has little influence on the isoelectric point of a protein. In effect, PromoFluor Maleimides have an advantage over amino group labels because they do not interfere with techniques like differential 2D gel electrophoresis. Since most proteins contain only a few thiol functional groups, thiol-reactive dyes can be used to selectively modify a protein or peptide containing a thiol group at a specific site. Moreover, these dyes are well suited for labelling oligonucleotides. Maleimides couple to thiol groups under very mild conditions at a pH around 7.