The NHS ester (or succinimidyl ester) is the most popular amine reactive group for labeling with the primary amines of proteins (Lys), amine-modified oligonucleotides, and other amine-containing molecules. Proteins can be labeled with AFDye™647 NHS Ester at high molar ratios without significant self-quenching, leading to brighter conjugates and more sensitive detection. AFDye™ 647 conjugates are often a probe of choice for detection of low abundance targets.