Lower expression of acetylated histone-3/histone-4 and higher HDAC activity characterize primary WM cells compared with healthy donors. Immunocytochemical analysis of primary CD19⁺ cells isolated from BM of 4 patients with WM (A) and CD19⁺ cells isolated from PBMCs of 4 healthy donors (B) was performed with the use of anti–acetyl-histone-H3 (Lys18), -H3 (lys9/14), and -H4 (lys12) antibodies. DAPI (4′-6′-diamidine-2-phenylindole) was used to stain nuclei. (C) HDAC activity was assessed with nuclear extracts with the use of a Colorimetric HDAC Activity Assay Kit on primary CD19⁺ cells isolated from BM of 6 patients with WM (WM1, WM2, WM3, WM4, WM5, WM6); primary CD19⁺ cells were isolated from PBMCs of 3 healthy donors (average HD), and BCWM.1 and low-grade lymphomas IgM-secreting cell lines (all P ≤ .05).