In Vitro: Guidelines (Following is our recommended protocol. This protocol only provides a guideline, and should be modified according to your specific needs).Fixation and staining of self-assembled spheroids: 1. Resuspended cells are stained with CellTracker Red CMTPX and allowed to self-assemble into spheroids before aggregation. 2. Cell aggregates are fixed in 4% paraformaldehyde/PBS for 30 min with gentle inversion, then washing cells in PBS for 3 times and stained with 100 ng/mL Hoechst dye overnight prior to imaging.