Manifestation of the fragile site Xq27 in fibroblasts. III. A method to demonstrate R-type replication patterns and the fragile site.

A protocol is reported which allows the efficient induction of bromodeoxyuridine (BrdU)-induced R-type replication patterns in fibroblast cultures prepared to demonstrate the fragile site fra(X)(q27). The technique includes partial synchronization of the culture by fluorodeoxyuridine (FdU) blocking at the G1/S transition. This block does not impair the induction of the fragile site in medium 199 containing methotrexate. The marked increase of the mitotic index in the synchronized culture may be an advantage in the study of folic acid sensitive fragile sites in fibroblasts. Adding BrdU after block removal leads to an efficient labeling of replicating chromosomes without severely impairing the manifestation of fra(X)(q27).