Chromosomal DNA is packaged into chromatin, a dynamic structure composed of proteins and DNA. For genes to be switched on, chromatin must be in a loosely packaged state, while the opposite is true for genes to be switched off. Chromatin remodelling complexes contain motor proteins that use chemical energy to alter chromatin structure and thereby switch genes on and off in the right cells and at the right time. Peter studies a chromatin-remodelling complex called SWI/SNF. One of its components, SNF5 is inactivated in malignant rhabdoid tumours and normally acts as a tumour suppressor, a protein that protects cells from becoming cancerous.