HDAC9 and proximal spinal muscular atrophy: Histone deacetylase inhibitors (HDAC): Histone deacetylase (HDAC) inhibitors have been extensively investigated for their role in treating spinal muscular atrophy (SMA) due to their ability to activate SMN2 transcription by inhibiting deacetylation of chromatin histones, thereby promoting gene expression [104,105,106,107].