Differing combinations of apoptotic mediators, cell cycle abnormalities and differentiation blocks between patient samples make it difficult to predict cellular outcome to HDAC inhibitor treatment, although previous studies have shown that monocytic AML cell types such as the M5 cell line THP1 have greater apoptotic sensitivity to the HDAC inhibitor vorinostat than other non-monocytic AML cell lines [24]; this has been potentially linked to activity of Bcl-2 family members [25]. This evidence concerns the gene HDAC9 and acute myeloid leukemia.