Figure 6: Relationship between cell activity and cell number in expanding cells. This is a strategy for ZYX Btr to harvest cells at their peak activity point. Many cells gradually lose their functions or activities (eg. CTL reduces its cytotoxicity, hematopoietic stem cell loses its engraftment potential) during their in vitro expansion. At early stage of the expansion, the cell number increases faster than the activity decreases, which results in total activity increases. Afterwards, the individual cell-based activity drops very fast and leads the total activity decreases. Therefore, the best cell harvest time should be at the time point right before the total activity decreases.