PDE4D and neurodevelopmental disorder: The importance of dimeric forms of PDE4D for normal brain function is underscored by the discovery of PDE4D missense mutations in an ultrarare, neurodevelopmental disorder known as acrodysostosis type 2 with or without hormone resistance (Lee et al., 2012; Linglart et al., 2012; Michot et al., 2012; Lynch et al., 2013).