The invention relates to histone deacetylase (HDAC) biomarkers in multiple myeloma. Specifically, the biomarkers are drug specific, histone deacetylase (HDAC) or HDAC6 biomarker peptides, which are acetylated, for multiple myeloma. Alternatively, the biomarkers are drug specific, HDAC6 biomarker peptides, which are acetylated or unacetylated, for multiple myeloma. The invention also relates to a kit comprising a detection agent and instructions for identifying a biomarker peptide of the invention. The invention further relates to a method for monitoring treatment efficiency of an HDAC inhibitor in a subject.