CHO cell lines were grown on coverslips and 24 h after doxycycline-induction, cells were washed with PBS and fixed with 4% PFA in DMEM for 30 min. The fixative was removed, and free aldehydes were quenched with 100 mM glycine in PBS. Then, samples were washed with PBS and permeabilized for 10 min with PBS containing 0.1% Triton X-100. Cells were incubated in blocking solution (2% bovine serum albumin, 0.2% Tween 20, 5% glycerol, and 0.05% sodium azide in PBS) and subsequently stained with a primary mouse-anti-flag antibody (1:400) and a secondary anti-mouse antibody coupled to rhodamine (1:200, Jackson Immunoresearch). Cell nuclei were stained with DAPI (49,69-diamidino-2-phenylindole, Sigma). Microscopic analysis was performed with a Zeiss ApoTome/ Axiovert 200 M microscope using a magnification of 1:40.