A comparison of 5-bromodeoxyuridine and thymidine incorporation into fertilized sea urchin eggs

³H-Thymidine (³H-TdR) and ³H-5-bromo-2-deoxyuridine (³H-BUdR) are first incorporated into newly fertilized Strongylocentrotus purpuratus eggs 30 min after fertilization, which is about the time of pronuclear fusion. An initial incorporation of ³H-BUdR immediately following fertilization has been previously reported. This was found to be due to a radioactive impurity present in samples of ³H-BUdR stored in aqueous solutions. ³H-TdR stored in water, or ³HBUdR stored in 50% ethanol, did not contain this impurity.