Miniature organisms are no longer the sole preserve of Japanese bonsai lovers. Ernst Hafen and colleagues (Cell 97, 865-875; 1999) have discovered that flies with mutations in a gene termed chico — meaning ‘small boy’ in Spanish — are less than half the normal size (compare the chico mutant fly with the wild-type fly below it). Not only are the cells in chico − flies smaller than usual, but there are fewer of them — defects that the authors attribute to decreased cell growth and proliferation rather than increased cell death.

The Chico protein is, in fact, very similar to the vertebrate IRS1-4 proteins, which are involved in cellular metabolism. Consistent with this, Hafen's group find that chico − flies also have metabolic defects — despite their diminutive stature, they contain twice as much lipid as normal. It also turns out that chico − flies are similar in size to normal flies reared without proper nutrition. So, the authors propose that Chico may belong to a nutritional sensing system which, by regulating cell proliferation, growth and metabolism, controls growth in response to nutritional conditions.