From The New England Journal of Medicine Birth defects, the leading cause of infant mortality in the United States, are responsible for nearly 20 percent of all infant deaths. In the 20th century, there was remarkable progress in cataloguing the chromosomal abnormalities, human teratogens, and single-gene defects that cause birth defects. Yet even today, the causes of two thirds of all birth defects remain unknown. The revolution in the biochemistry of genetics opened a window on the ways in which metabolic errors can lead to diverse clinical syndromes such as the mental retardation caused by phenylketonuria. That breakthrough led to the recognition that, when it is begun days after birth, dietary treatment for phenylketonuria can prevent mental retardation. The discovery marked the beginning of newborn screening, one of the most successful public health programs in history. We are now on the brink on another revolution -- the understanding of the molecular basis of abnormal morphogenesis and the promise of finding ways to treat and prevent birth defects. During the past three decades, advances in molecular genetics have led to remarkable progress in our understanding of the molecular basis of morphogenesis. As with the biochemistry of genetics, specific inborn errors of morphogenesis have provided a window through which we can understand normal morphogenesis at the molecular level. This book, to my knowledge, is the first comprehensive review of abnormal and normal morphoge