The studies of cell growth and division have remained at the centre of biomedical research for more than 100 years. The combination of genetic, biochemical, molecular and cell biological techniques recently yielded a burst in what is known of the molecular control of cell growth processes. The initiation of DNA replication is crucial for the stability of the genetic information of a cell. Two factors, Cdc45p (cell division cycle 45p) and DNA polymerase a-primase, are necessary in this process. Depending on growth signals, Cdc45p is expressed as a late protein. New phosphorylation-specific antibodies specifically recognize the phosphorylated subunit, p68, of the four subunit DNA polymerase a-primase and show that the phosphorylated polypeptide is exclusively nuclear.