tokens: Following washing and 2 minute ' s incubation with 0 . 1 M Tris to neutralize the acid , cells were re - suspended in FACS buffer ( 0 . 5 % fetal calf serum in PBS ) and stained with Phycoerythrin ( PE ) - conjugated anti - BrdU according to the manufacturer ' s protocol for flow cytometry analysis on a FACScan instrument .