Eichele G
Trends Genet. 1989 Aug;5(8):246-51. doi: 10.1016/0168-9525(89)90096-6.
It has long been suggested that pattern formation depends in part on signalling molecules known as 'morphogens', diffusible substances that determine cell fate in a concentration-dependent way. Retinoic acid, a small hydrophobic molecule that binds to nuclear receptors, is a candidate morphogen for specifying the anteroposterior pattern of vertebrate limbs.