In addition, I looked into RNA-binding domains (RBDs) with well-defined RNA sequence specificity. I examined the precursor mass shifts from peptides located within PABPC1 RRM1-2 (poly(A)-binding) , RBFOX2 RRM (G-crosslinking) , and PTBP1 RRM1 (polypyrimidine-binding) . Most unexpectedly, methyladenine/adenosine crosslinks , but not adenine/adenosine crosslinks, were detected on H144 of PAPBC1 RRM2 domain (Figure ). Meanwhile, guanosine crosslinks were identified on F154/V155 of RBFOX2 RRM (Figure ), and cytosine crosslinks on H133 of PTBP1 RRM1 (Figure ). All identified RBSs (except V155, which may have been catalogued owing to in-solution dynamic fluctuation, mislocalization during open search, or electron hopping during UVCcrosslinking ) were proximal to adenines, guanine, or cytosine in previously reported ribonucleoprotein structures , respectively. RBSs harboring the five newly identified RNA adducts were predominantly identified in annotated RBPs (Table ). These adduct types may be incorporated in closed search parameters on UVC-crosslinked RBS datasets, but at the cost of search space inflation . (amino acid positions: 11-175) . Bottom: Structure of PABPC1 RRM1-2 in complex with poly(A) RNA (PDB: 1CVJ) . (b) UVC-crosslinked guanosine on RBFOX2 RRM. Top: