As January's gloom dissipates, we can look forward to a little more daylight. In a similar vein, two articles in our February issue help to shed light on two different biological networks. Deciphering these complex systems is a laborious process, but one that can ultimately provide a wealth of information about many disease processes as well as normal physiology.

On page 85, Cullen M. Taniguchi, Brice Emanuelli and C. Ronald Kahn discuss the concept of 'critical nodes' as a means of elucidating the complexity of cell-signalling networks. The authors propose three key criteria for defining the critical nodes in any particular ligand–receptor system and, using the insulin-signalling pathway as a model system, reveal some of the critical nodes that are involved in insulin signalling.

Another complex network is involved in the epithelial–mesenchymal transition (EMT). This process is essential for the formation of tissues and organs, and unravelling how it is regulated could have important implications for the study of diseases such as fibrosis and cancer. Jean Paul Thiery and Jonathan P. Sleeman (page 131) dissect recent insights into the signalling strategies and the molecular basis of EMT, and reveal that EMT is regulated by a complex system of intracellular pathways with extensive crosstalk and many common endpoints.

And finally, as a last salute to the dying days of winter, we turn to cell death in our Article Series on Developmental Cell Biology (http://www.nature.com/nrm/series/devcellbiol). Guillaume Lettre and Michael O. Hengartner (page 97) focus on recent, exciting findings in the regulation and execution of developmental apoptosis in the simple nematode Caenorhabditis elegans, which are providing insights into apoptosis in more complex species.