Strategy for N-CLAP. After PITC treatment, all amines in a protein are blocked (filled circle). Reaction with TFA selectively deblocks the N terminus but not PITC-modified lysines. The newly generated amino groups at the N-termini are then reacted with an amine-specific labeling reagent, such as EZ-Link Sulfo-NHS-SS-biotin (oval). After protein digestion, the N-terminal peptides are recovered using avidin-based resins, eluted with a reducing agent, and then identified by MS.