Fig. 9. The roles played by three transcription factors involved in cell cycle regulation in Drosophila melanogaster. The E2F/DP and DREF transcription factors have been shown to promote DNA replication (progression from G₁ into S) in mitotic and endocycling cells. Data from previous (Fung et al., 2002; Katzen et al., 1998) and current investigations of the function of Dm myb have demonstrated that although in mitotic cells, DMyb shares the function of being a positive regulator of progression from G₁ into S with E2F/DP and DREF, DMyb also acts as a negative regulator of endoreduplication and is able to promote progression from G₂ into M. We propose that the ability of DMyb to inhibit endoreduplication is an important aspect of maintaining genomic integrity in proliferating cells. We have also shown that while the abilities of full-length and truncated (activated) DMyb proteins to promote cell cycle progression in proliferating cells were similar, the truncated protein was considerably more potent at inhibiting S phase in endoreduplicating cells. As indicated in the figure, we hypothesize that one or more factors, to which we refer as DMyb activating factors (DMyb AF), must interact with full-length DMyb to activate its potential as a transcriptional regulator, and that these factors are present in proliferating cells, but absent in endocycling cells.