Plakoglobin is the only component common to both the desmosomal plaque and the cadherin-catenin cell adhesion complex in the adherens junction. It is highly homologous to vertebrate beta-catenin and to Drosophila armadillo protein and may-like these proteins-be also involved in signaling pathways. To analyze the role of plakoglobin during mouse development we inactivated the plakoglobin gene by homologous recombination in embryonic stem cells and generated transgenic mice. Plakoglobin null-mutant embryos died from Embryonic Day 10.5 onward, due to severe heart defects. Some mutant embryos developed further, especially on a C57BL/6 genetic background, and died around birth, presumably due to cardiac dysfunction, and with skin blistering and subcorneal acantholysis. Ultrastructural analysis revealed that here desmosomes were greatly reduced in number and structurally altered. Thus, using reversed genetics we demonstrate that plakoglobin is an essential structural component for desmosome function. The skin phenotype in plakoglobin-deficient mice is reminiscent of the human blistering disease, epidermolytic hyperkeratosis.