Hedgehog signaling: from the Drosophila cuticle to anti-cancer drugs.

Hh signaling controls cell proliferation and differentiation in processes that range from insect segmentation and limb formation to vertebrate neural tube development and bone differentiation. Moreover, Hh signaling appears to regulate stem cell homeostasis in adult tissues, while persistent Hh pathway activity has pathological consequences in a number of cancers. Two recent meetings, a Karolinska Institute Nobel conference (August 22-24, 2004) and a joint EMBO and Juan March Institute workshop (October 25-27, 2004), provided the opportunity to take stock of the progress that has been made in understanding Hh signaling and also to remind us of the many questions that still remain unanswered.