The structure of the X (or ADRP) domain of a pathogenic variant of feline coronavirus (FCoV) has been determined in tetragonal and cubic crystal forms to 3.1 and 2.2 ร… resolution, respectively. In the tetragonal crystal form, glycerol-3-phos-phate was observed in the ADP-ribose-binding site. Both crystal forms contained large solvent channels and had a solvent content of higher than 70%. Only very weak binding of this domain to ADP-ribose was detected in vitro. However, the structure with ADP-ribose bound was determined in the cubic crystal form at 3.9 ร… resolution. The structure of the FCoV X domain had the expected macro-domain fold and is the first structure of this domain from a coronavirus belonging to subgroup 1a. ยฉ 2009 International Union of Crystallography.