Fig. 1. Transcriptional activity of the exogenous cyclin B1 promoter is high in the G₂/M phases of the cell cycle. (A) DNA distribution analysis of propidium iodide-stained asynchronous (Async.) and MSO HeLa cells. (B) Northern blot analysis of CAT mRNA levels in asynchronous (Async.) and MSO cells. HeLa cells stably transfected with the CAT reporter gene fused to the 332 bp cyclin B1 promoter were synchronized by a double thymidine block. The filter was hybridized with CAT, murine cyclin B1 and human GAPDH cDNAs.