Replication patterns of the fragile X in heterozygous carriers: analysis by a BrdUrd antibody method.
 The replication status of the fragile X chromosomes was studied in short-term cultures of lymphocytes from six female heterozygous carriers.
 The fragile X was induced by adding 0.1 microM fluorodeoxyuridine during the last 24 h of culturing.
 The replication status of the X chromosomes was studied using a bromodeoxyuridine (BrdUrd) antibody method.
 BrdUrd was added (1) at a final concentration of 0.2 micrograms/ml during the early S phase of chromosome replication (16-10 h before harvest), (2) at 0.2 microgram/ml during the late S phase (the last 6 h of culturing), (3) at 20 micrograms/ml during the early S phase, and (4) at 20 micrograms/ml during the late S phase.
 BrdUrd that was incorporated into replicating chromosomes was detected by using a nuclease and BrdUrd monoclonal antibody.
 The frequency of the fragile X was reduced by BrdUrd treatment.
 The degree of reduction was more severe in the 20 micrograms/ml than in the 0.2 microgram/ml series and was more severe with late S than with early S treatment.
 Of the early- and late-replicating fragile X chromosomes, those which were actively replicating during a BrdUrd treatment were more reduced than the others.
 Thus, the average rate of early and late S treatment with 0.2 microgram BrdUrd/ml was assumed to be the closest reflection of the situation in vivo.
 There was no correlation between the average rate of the early replicating, active fragile X and the intelligence of the heterozygous carriers studied.
