Post-translational ubiquitin-protein conjugates are recognized for degradation by the ubiquitin fusion degradation (UFD) pathway. Several proteins involved in this pathway have been identified. This family includes UFD1, a 40kD protein that is essential for vegetative cell viability. The human UFD1 gene is expressed at high levels during embryogenesis, especially in the eyes and in the inner ear primordia and is thought to be important in the determination of ectoderm-derived structures, including neural crest cells. In addition, this gene is deleted in the CATCH-22 (cardiac defects, abnormal facies, thymic hypoplasia, cleft palate and hypocalcaemia with deletions on chromosome 22) syndrome. This clinical syndrome is associated with a variety of developmental defects, all characterized by microdeletions on 22q11.2. Two such developmental defects are the DiGeorge syndrome OMIM:188400, and the velo-cardio- facial syndrome OMIM:145410. Several of the abnormalities associated with these conditions are thought to be due to defective neural crest cell differentiation.