Biochemical Reagents, CEP, Protein-RNA binding click probe, CAS 1838570-66-3, 98% by TLC/NMR (Conforms), CEP (1838570-66-3) is a clickable electrophilic purine which may be used to directly quantify protein-RNA interaction on proteins via photoaffinity competition with 4-thiouridine-labeled RNA in cells. Employing CEP in a photo-activatable-competition and chemoproteomic enrichment (PACCE) protocol enables functional profiling of canonical RNA-binding domains as well as the discovery of “moonlighting” RNA binding activity in the human proteome. This new methodology represents a chemoproteomic platform for global quantification of protein-RNA binding activity in living cells.