UPF3B and cask-related x-linked intellectual disability: These residues are important because UPF3B Y160D is implicated in XLID disease phenotypes (25,26,28), and UPF3B residues Y160 and Y167 can be phosphorylated in human cells (58,59) and also in our recombinant UPF3B expressed in insect cells (Supplementary Figure S10).