Treatment with the HDAC inhibitor valproate restored the expression of 1533 genes down-regulated at pH6.8, including 962 cellular process-related genes, 810 organelle-related genes, 707 metabolic process-related genes, 483 binding-related genes, 310 membrane-enclosed lumen-related genes, and 93 catalytic activity-related genes, indicating the important role of histone deacetylation in gene repression in MM cells under acidic conditions. Here, HDAC9 is linked to Miyoshi myopathy.