DXO is a multifunctional RNA processing enzyme that serves as a quality control mechanism for RNA cap structures in mammalian cells. Its primary function involves decapping activity against non-canonical RNA caps, specifically removing NAD+ caps from the 5'-end of RNAs through a 'deNADding' reaction 1. Unlike the canonical m7G cap which promotes RNA stability, NAD+ caps promote mRNA decay, and DXO efficiently removes these caps to regulate cellular RNA levels 1. DXO also exhibits decapping activity toward FAD-capped and dpCoA-capped RNAs 2. The enzyme functions as a pre-mRNA capping quality control system by degrading incompletely capped RNAs, showing activity against unmethylated 5'-end-capped RNA (cap0) while sparing properly modified 2'-O-methylated m7G caps (cap1) 3. DXO possesses both decapping and 5'-3' exoribonuclease activities, enabling it to completely degrade faulty RNA molecules 3. Located in the MHC class III region, DXO is part of an RNA metabolism and surveillance quartet critical for transcriptional and translational processes 4. The enzyme shows nuclear localization and may have clinical relevance, as it has been associated with immune-mediated diseases including celiac disease 5.