Our cover this month is inspired by what are arguably the most beautiful cells in the immune system — dendritic cells. As Ken Shortman and Yong-Jun Liu discuss in their Review on page 151, dendritic cells regulate crucial decisions in innate and adaptive immunity. In terms of function, phenotype and location they comprise a very heterogeneous class of cells, and the relationship between different dendritic cell subsets has proved difficult to resolve. Here, the authors draw on recent findings in mouse and human to show that the development of dendritic cells is likely to incorporate features from two models of development — the multilineage model and the functional subsets model.

Developmental decisions in the lymphoid lineage are the focus of Katia Georgopolous' Review on page 162. Chromatin codes, chemical modifications that affect chromatin structure and the position of genetic loci in the nucleus are thought to direct and perpetuate cell-fate choices during haematopoiesis. The pivotal role of Ikaros, a zinc finger DNA-binding protein, in this epigenetic decision making is discussed.

Decisions of another kind — ethical ones — are discussed in a Science and Society article by Anthony Fauci and colleagues, on page 210. Clinical trials conducted in developing countries by organizations from developed nations present an ethical minefield, and current guidelines put in place to prevent exploitation dictate that the subjects of such trials should receive a standard of care equivalent to that expected in developed nations. But does this ultimately benefit those in greatest need? Using the example of research into HIV/AIDS therapy, the authors argue that this well-intentioned restriction might actually undermine biomedical research aimed at improving global health.