RNA decapping enzymes catalyze the removal/degradation of the 5' cap structure that is present on most eukaryotic mRNAs and small nuclear RNAs, and thereby modulate RNA turnover. Canonical decapping enzymes (Nudix hydrolases, such as DCP2) hydrolyse a pyrophosphate bond in the 5' N7-methyl guanosine (m7G) cap of intact RNA molecules, thereby exposing a free 5' monophosphate to nucleases and initiating a 5' -> 3' decay pathway. Others (HIT family proteins, such as DCPS) act as scavengers and cleave a pyrophosphate bond of m7G capped di- or oligonucleotide remnants of the 3' -> 5' decay pathway. Members of the diverse DXO protein family (e.g. Rai1/DXO) are mainly involved in mRNA cap quality control and selectively cleave bonds in the 5' end of uncapped (triphosphorylated) mRNAs or mRNAs with an unmethylated cap. DXO family proteins are also involved in the removal of the NAD+ cap variant. (Adapted from PMID:30345629.)