A modified cell cycle, the endo cycle, produces the polyploid or polytene cells that are present in some tissues of most organisms. In the endo cycle, the S phase alternates with a gap phase, but mitosis does not occur. Genes needed to inhibit mitosis during the endo cycle and to promote the onset of S phase have been identified in Drosophila. Genomic intervals are differentially replicated during the endo cycle S phase such that some regions are under-replicated, while others can be amplified. Cyclin E and E2F are needed for this differential DNA replication during Drosophila oogenesis.