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Cell-cycle fractionation by cyclin B1 flow cytometry. Human lymphoblastoid cells were labeled with BrdUrd for 90 min (5), fixed, and separated on the basis of DNA content and cyclin B1 levels. The cyclin B1 units are arbitrary. Equal numbers of cells (between 5,000 and 10,000) were collected from each sorting fraction (G1; S1–S4; and low, mid-, and high cyclin B1) and used for subsequent analysis of replication timing (modified from ref. 9).
