text: Due to macromolecular crowding, enzyme assays and biophysical measurements performed in dilute solution may fail to reflect the actual process and its kinetics taking place in the cytosol. One approach to produce more accurate measurements would be to use highly concentrated extracts of cells, to try to maintain the cell contents in a more natural state. However, such extracts contain many kinds of biologically active molecules, which can interfere with the phenomena being studied. Consequently, crowding effects are mimicked in vitro by adding high concentrations of relatively inert molecules such as polyethylene glycol, ficoll, dextran, or serum albumin to experimental media. However, using such artificial crowding agents can be complicated, as these crowding molecules can sometimes interact in other ways with the process being examined, such as by binding weakly to one of the components.