ASCC1 and proximal spinal muscular atrophy: In addition, mutations in Thyroid Hormone Receptor Interactor 4 (TRIP4) and Activating Signal Cointegrator 1 Complex 1 (ASCC1), which are components of the RNAP II/U1 snRNP machinery, are causes of a prenatal form of SMA (Table 1) (32,33).