OGT and X-linked intellectual disability: Recently, ogt missense mutations associated with X-linked intellectual disability have been identified.54 These occur in the TPR domain of OGT54–56 as well as in the catalytic domain.57,58 Some of these mutations cause a decrease in endogenous OGT activity.55,57 Therefore, it is possible that abolished O-GlcNAcylation status on a subset of OGT substrates or even a single substrate/site could contribute to the intellectual disability phenotype, an avenue that is currently being explored in our laboratory using several methods discussed in this review.