Very little is known about the expression patterns in plants of genes that encode proteins involved in the initiation of DNA replication. Partial cDNA sequences that encode Cdc6 and Mcm3 in tobacco have been isolated. The sequences were used as probes in northern blots which suggested that, in the cell cycle of synchronized tobacco BY-2 cells, expression of CDC6 is confined to late G(1) and S-phase whereas the expression of MCM3 is not confined to any particular cell cycle phase. These data were confirmed and extended by real-time PCR measurements of mRNA abundance through the cell cycle. CDC6 exhibits a very clear peak of expression in S-phase whereas MCM3, expressed at a much lower level than CDC6, is not cell-cycle-regulated. These patterns of cell cycle gene expression resemble those found in the fission yeast Schizosaccharomyces pombe rather than those in budding yeast or mammalian cells.