5-bromodeoxyuridine (5-BUdR), a thymidine analog, is incorporated into DNA during replication. When both DNA strands contain 5-BUdR, the chromatid stains less intensely than when only one DNA strand contains the analog. When cells are grown in the presence of 5-BUdR for two replication cycles, the two sister chromatids stained differentially are called harlequin chromosomes.