The aim of this experiment was to identify the catalytic cysteine in the antiviral E3 ligase ZNFX1 using activity-based probe (ABP)–enabled cross-linking mass spectrometry (XL-MS). A biotinylated E2~ubiquitin activity-based probe containing a cysteine-electrophile trap was used to covalently capture ubiquitin a transfer intermediate between UBE2D2~Ub and full-length human ZNFX1. Cross-linked peptide mapping was performed to identify probe-labelled residues and putative catalytic site.