SPIM imaging of spheroids of HCT116 colon carcinoma cells expressing a histone H2B-HcRed fusion protein. A: A frozen section from the center of an H2B-HcRed-expressing HCT116 cell spheroid. DNA is stained with DAPI. Scale bar, 100 μm. B: XY planes at the indicated z positions of the 200 μm z-stack shown in Additional file 4 (10× objective, NA = 0.3). C: Region of the XY plane at 50 μm in depth in the z-stack (left). The dashed line indicates the Y position of the XZ plane (right), parallel to the detection axis. Scale bar, 100 μm. D: 3D visualization of the z-stack presented in the Additional file 4.