On the cover: Double outlet right ventricle in Tbx3 mutant embryonic mouse hearts. Three-dimensional reconstructions of the blood-filled lumen show the right (blue) and left (red) ventricles and outflow tract vessels in a wild-type embryo (left) and a Tbx3 mutant embryo (right) at 13.5 days of embryonic development. In mutants, both the aorta and the pulmonary trunk originate from the right ventricle. Note the interventricular communication between the left and right ventricles in the mutant. See related article, page 1340.