PDE4D and acrodysostosis: Thus it remains to be discovered whether long form monomer–dimer equilibrium can be altered by the action of various kinases able to phosphorylate residues within UCR1 and the PDE4 catalytic unit [23–26,28–30,70,71]; through mono-ubiquitination by the β-arrestin-sequestered E3 ligase, Mdm3 [32]; as a consequence of mutation of residues in UCR1 of PDE4D as seen in acrodysostosis (e.g. Q164P and L166S in PDE4D5; [72]) and when interacting with various partner proteins that sequester PDE4 isoforms to underpin compartmentalised cAMP signalling [1].