Put that hunchback over here

Fly embryo development is the classic example of refinement. From initial gradients of proteins like Bicoid (Bcd) come first the broad domains of gap gene expression and, finally, the narrower segmentation patterns. Now, Bahram Houchmandzadeh, Eric Wieschaus, and Stanislas Leibler (Princeton University) have looked at just when this cascade establishes precision. They find that precision is initiated not gradually but suddenly and early, with the onset of expression of the gap gene hunchback (hb).

Perhaps not surprisingly, the Bcd gradient itself shows significant variation. A particular threshold level of Bcd occurs across a range of 30% of the embryonic length in different embryos. Hb, however, is turned on so precisely that in two-thirds of the embryos the Hb boundaries are defined over a region corresponding to less than the...