Scratching the Bose surface.

Particles known as fermions (such as electrons) obey the Pauli exclusion principle, which decrees that only a single fermion can occupy a particular state, such as an orbital in an atom. But the second family of particles, bosons (such as helium atoms), have no corresponding restrictions on their occupation of states. This distinction in their individual properties also carries over to the collective properties of large numbers of fermions or bosons at low temperatures. The typical quantum state of many fermions is a Fermi liquid, formed, for example, by the valence electrons in all metals: the electrons can move from atom to atom in this state and conduct electricity across macroscopic distances, albeit with a finite resistance. In contrast, bosons typically form a Bose–Einstein condensate, which is the basis of superfluidity in helium and its ability to flow perpetually with negligible dissipation. Figure 1 Taking steps to developing a leg. a, The Wingless (Wg) and Decapentaplegic (Dpp) proteins are expressed in wedges in the fly leg disc, which is set aside early in development. b, Wg and Dpp activate the Dll (red) and dac (green) genes at two different positions along the proximal–distal (P–D) axis. Proximally expressed gene-transcription factors are shown in dark blue. c, Growth of the leg disc creates a new domain in which Dll and dac expression overlaps (yellow). Vein and Rhomboid — which provide a source of ligands for the epidermal-growth-factor receptor (EGFR) — are activated by Dpp and Wg at the distal tip (grey spot). d, The source of EGFR ligands (grey spot) results in a gradient of EGFR activity (graded grey shading) that provides positional information for new domains of gene expression (purple and light blue). Cross-regulation by some of these factors (black bars) also contributes to the definition of these domains, which are shown as different shades of orange, purple and blue. Details are given in the new papers. e, A mature leg is patterned along its proximal–distal axis by a combination of the activity gradients of Dpp plus Wg, and EGFR.