SAHA-d5 contains five deuterium atoms at the 2, 3, 4, 5, and 6 positions. It is intended for use as an internal standard for the quantification of SAHA by GC- or LC-MS. SAHA is a histone deacetylase (HDAC) inhibitor that binds directly to the catalytic site of the enzyme thereby blocking substrate access. It inhibits class I, II, and IV HDACs at 50-200 nM and arrests cell growth in a wide variety of transformed cells in culture at 2.5-5.0 µM.