In a new study published in Disease Models & Mechanisms, scientists use the zebrafish to gain insight into the influence of known cancer genes on the development and progression of melanoma, an aggressive form of human skin cancer with limited treatment options.

If we imagine our immune system to be a police force for our bodies, then previous work has suggested that the Lymph nodes would be the best candidate structures within the body to act as police stations – the regions in which the immune response is organised. However, Prof. Burkhard Becher, University of Zurich, suggests in a new paper – published in this week's issue of PLoS Biology – that lymph nodes are not essential in the mouse in marshalling T-cells (a main immune foot soldier) to respond to a breach of the skin barrier. This result is both surprising in itself, and suggests a novel function for the liver as an alternate site for T-cell activation.

Even microbes are governed by the principle of supply and demand – at least at the genetic level. Not all of their gene products, the blueprints for proteins, are required at all times. That means most of their genes only become active when they are needed, as is the case in higher organisms. In the simplest case, a transcription factor will activate the gene in question at the right time. Genes that are regulated in a somewhat more complex manner, on the other hand, are kept inactive by a repressor that is removed only when the gene is needed. Which of these two regulation mechanisms will develop is a question of demand, along the lines of a "use-it-or-lose-it" principle: if genes are frequently active, then, as a rule, they will be directly induced. Genes that encode more rarely used proteins, on the other hand, tend to be kept inactive by repressors. LMU physicist Ulrich Gerland and Professor Terence Hwa of the University of California have now demonstrated using computer simulations and theoretical analyses that another – indeed opposing – principle also comes into play: "wear-and-tear". According to this principle, direct activation can lead to harmful changes. "Which of the two principles prevails depends on evolutionary criteria such as the population size and the periods over which environmental changes take place," says Gerland. "Our study may serve as a useful basis for more detailed studies of the evolution of regulatory systems." (PNAS Early Edition, 22 Mai 2009)