Together with other volumes in this series, Volume 53 presents thoughtful and forward-looking articles on developmental biology and developmental medicine. The exceptional reviews in this volume of "Current Topics in Developmental Biology" will be valuable to both clinical and fundamental researchers, as well as students and other professionals who want an introduction to current topics in cellular and molecular approaches to developmental biology and clinical problems of aberrant development. This volume in particular will be essential reading for anyone interested in stem cells, signaling, medical implications of developmental determinants, hematopoeisis, axis specification, and molecular genetics of development. It includes a landmark review on Hedgehog genes and their gene products in invertebrate and vertebrate development. It presents major issues and astonishing discoveries at the forefront of modern developmental biology and developmental medicine. It is the longest-running forum for contemporary issues in developmental biology with over 30 years of coverage.