For less specialized cultures, a common practice is to use a general medium, such as DMEM, and supplement it with factors that support the cells. For example, many researchers supplement their basic DMEM with fetal bovine serum (FBS). “Serum provides a lot of nutrients as well as certain attachment factors,” says George Sitterley, a market segment manager for research cell culture at Sigma-Aldrich. Experimenting with serum levels is smart, because some cells prefer more serum than others. “We recommend a starting percentage of 10% FBS, but you can increase or decrease the percentage based on cell viability,” says Tabitha Eckert, custom monoclonal team leader at Rockland Immunochemicals. “At Rockland, we look for media that are formulated to contain high glucose and also L-glutamine, but not all DMEMs contain these ingredients at the same amount.” Cells use glucose and glutamine as energy sources, but their preference for amounts can vary by cell type. “Some cells prefer more glucose than others,” says Sitterley. “Glutamine is usually very important, so we offer media with and without it to allow people to modify the glutamine concentrations. Even within a normal glutamine range, certain cells will like more glutamine than others.”