The observation that two different components of the ASC-1 complex are mutated to cause SMA and that the ALS-causative proteins mediate the association of the ASC-1 complex with RNAP II provide excellent examples of the importance of identifying interaction partners of ALS/SMA-causative proteins, as these interaction partners themselves are candidates for causing the diseases. Here, TRIP4 is linked to proximal spinal muscular atrophy.