The aim of this review is to summarize some of the recent advances in molecular embryology that help to explain the pathogenesis of holoprosencephaly (HPE), or its related malformation in model organisms, cyclopia, and laterality defects in humans, derived from detailed analysis of similar malformations in animal models. Recently, defects in several developmental pathways including those operated by the Sonic hedgehog and Nodal signaling factors have been implicated as causes of HPE or laterality defects in humans. Here we summarize the (...) findings in animal models that indicate that both defects can be explained by mechanisms that relate to the proper development of the axial midline in vertebrates. BioEssays 23:888–900, 2001. Published 2001 John Wiley & Sons, Inc. (shrink)