This is a phase 1a/1b, multicenter, single-arm, open-label, dose escalation study to determine the maximum tolerated dose (MTD) and evaluate the safety and preliminary antitumor activity of ACY-241 for oral administration as monotherapy and in combination therapy with orally administered pomalidomide and low-dose dexamethasone in eligible patients with relapsed or relapsed-and-refractory multiple myeloma (MM).