Cells can build highly complex intracellular structures from small protein subunits. We study the cell biological mechanisms by which cells assemble a contractile actomyosin cytoskeleton and connect it to surrounding tissues. Our current research focuses on how myofibrils assemble in the fruit fly Drosophila melanogaster. We use Drosophila as a model system because of its unique genetics, which allows us to study the relationship and function of genes in tissues, and because of the high homology of basic cellular processes between Drosophila and humans. Myofibrils (see Figure) consist of an actin cytoskeleton arranged into a highly repetitive and regular, almost crystalline array of sarcomeres. Even small changes in sarcomere structure are readily visible in the microscope, making this an ideal model to study mutations in cytoskeletal proteins. We also study the function of some prominent muscle proteins in non-muscle cells. Our research leads to a better understanding of muscle disorders and their underlying cell biological defects.