Determining the abundance of a cellular protein is fundamental to understanding the cellular biology being studied. For cell-based assays, comparing the relative abundance between different experimental conditions is often the goal as opposed to measuring absolute protein levels. Many methods exist for quantitating cellular protein abundance, including colorimetric, fluorescent and bioluminescent assays. These approaches are often based on gel electrophoresis separation followed by staining or antibody-based western blotting. The ideal detection method is determined based on factors such as available antibodies, required quantitative dynamic range, throughput needs and protein expression level. The sensitivity of a method needs to be matched to the protein being detected. For example, proteins expressed at higher levels in cells can be measured using less sensitive colorimetric methods. However, for low-abundance proteins, bioluminescent methods that can quantitate down to nanomolar amounts may be necessary. By considering these factors, you can determine which experimental methods are best suited to quantitating cellular protein expression levels.