Comparing the robustness of ten different oscillator models, Wolf et al. concluded that negative feedback-based oscillators are more robust. They further suggest that, in Goodwin-like negative feedback loops with different numbers of intermediates, positive feedback makes the system less robust, in contradiction to Tsai et al.. However, while the size of the parameter region of oscillatory behavior is used to measure robustness in, local period sensitivity is the robustness metric in, which might explain the discrepancy. Wolf et al. show, nevertheless, that lengthening the negative feedback loop can improve robustness, a mechanism that has also been shown to reduce the cooperativity requirement. Kholodenko showed that oscillations are possible in the MAP kinase signaling pathway with a negative feedback from the final product to the first kinase in the chain. In this system, MAPK phosphorylation cascade provides the cooperativity that along with the negative feedback produces oscillations as discussed throughout this paper. Interestingly, the number of levels in the phosphorylation cascade determines the degree of cooperativity within the structure. We explore the implications of our insights in the context of the cellular circadian oscillator in mammals. Circadian clocks in other eukaryotes consist of similar components and interactions and thus, the following discussion is applicable to those organisms as well. The cell-autonomous circadian oscillator consists of certain ��core-clock�� genes, per and cry, that are transcribed, translated, and finally inhibit their own transcription. Delays in this feedback are due to cellular Formoterol Hemifumarate processes, such as posttranslational modifications, complex formation and BRD73954 nuclear transport. As seen in the figure, the core feedback loop resembles the core Goodwin motif and this feature was exploited as such in several early iterations of circadian oscillator models. While this core negative feedback loop involving only per and cry is potentially capable of producing oscillations, transcriptional repression that closes the loop would need to have a very high cooperativity to be capable of sustained oscillations.