Replication timing in different human artificial chromosomes. (a-c) Detection of BrdU (red) on artificial chromosomes (small arrows; larger version in inserts). (a) In mid S phase, at 2-4 h, two artificial chromosomes in line 17-D34 are BrdU positive. (b) The artificial chromosome in line 17-E29 is replicating early in S phase, in the 0-2 h period. (c) In mid S phase (2-4 h), three artificial chromosomes are being coordinately replicated in this spread from line 17-C20. Images shown are from the first half of S phase, and, as expected, Yq (arrowhead) is not replicating at this time.