On the cover: Frontal views of two littermate mouse embryos at embryonic day E14.5. The mouse on the left is wildtype in genotype, and the right embryo lacks Pbx1, a homeodomain transcription factor. Deletion of Pbx1 produces cardiovascular defects, which contribute to the generalized edema observed in these mice. Ten different combinations of null alleles in Pbx1, Pbx2, and Pbx3 show a spectrum of cardiac defects, representing how multigenetic inheritance can determine the manifestation of heart abnormalities. See related article, page 702.