Covalent modification by SUMO polypeptides, or sumoylation, is an important regulator of the functional properties of many proteins. Among these are several proteins implicated in human diseases including cancer, Huntington's, Alzheimer's, and Parkinson's diseases, as well as spinocerebellar ataxia 1 and amyotrophic lateral sclerosis. Recent reports reveal two new examples of human disease-associated proteins that are SUMO modified: amyloid precursor protein and lamin A. These findings point to a function for sumoylation in modulating amyloid-beta peptide levels, indicating a potential role in Alzheimer's disease, and for decreased lamin A sumoylation as a causative factor in familial dilated cardiomyopathy.