PDE4D and atypical Rett syndrome: Our previous study showed that miR-199a positively controls mTOR signaling activity by suppressing mTOR signaling inhibitors such as Pde4d, Sirt1, and Hif1α downstream of methyl-CpG-binding protein 2 (MeCP2), which is a causative gene for the severe neurodevelopmental disorder Rett syndrome (Tsujimura et al., 2015).