Genetic studies of Drosophila eye imaginal disc development have provided a classic approach to dissect the coordination of cell proliferation and differentiation. This paper describes the characterization of a mutation in the tenured gene, which was recovered from a large-scale genetic screen based on its glossy eye phenotype. The authors show that this phenotype arises from a specific block in the G1 phase of the cell cycle, with no apparent effects on cell viability or differentiation. The remarkable part of this paper lies in the discovery that tenured encodes a subunit of cytochrome c oxidase, a key component in the electron transport chain. The authors show that the reduced ATP levels in tenured mutant cells leads to activation of the energy sensor AMPK that, in turn, acts through a p53 signaling pathway to reduce levels of the cell-cycle regulator Cyclin E. This paper provides, for the first time, a link between cellular energy levels and cell-cycle progression, giving us a framework to understand how metabolism and proliferation are coordinated during development.This PaperPick refers to “Mitochondrial Regulation of Cell Cycle Progression during Development as Revealed by the tenured Mutation in Drosophila,” by S. Mandal, P. Guptan, E. Owusu-Ansah, and U. Banerjee, published in December 2005.Utpal Banerjee discusses the findings of his group's 2005 study and the progress that has been made since.