PPIL1 and pontocerebellar hypoplasia: Based on the genes involved, the underlying etiology of PCH can be further divided into three groups: tRNA-processing genes (CLP1, RARS2, SEPSECS, TSEN2, TSEN15, TSEN34, TSEN54), non-tRNA-processing genes targeting other forms of RNAs (CDC40, EXOSC1, EXOSC3, EXOSC8, EXOSC9, PTOE1, PPIL1, PRP17), and genes which are not directly involved in any form of RNA processing (VRK1, RDM13, AMPD2, CHMP1A, COASY, MINPP1, PCLO, SLC25A46, TBC1D23, VPS51, VPS53).