NAD is a noncanonical mRNA cap that challenges our traditional dogma of N7-methylguanosine (m7G)-capped eukaryotic mRNAs. The relationship between NAD and m7G caps has been elusive. Xiao et al. find that the deNADding enzyme DXO promotes maturation of m7G caps, suggesting that DXO fine-tunes the dynamic balance between alternative RNA cap structures.