We recommend assessing cell viability and S4U incorporation rates for all new cell types or experimental set-ups, using a time period twice the duration of the intended experiment. For example, if you wish to sample cells over a 6-hour period of S4U labeling, then cell viability should be assessed for a 12-hour duration.
NOTE: For longer labeling experiments (> 3 hours) we recommend replacing S4U-containing media every 3 hours.