Eukaryotic cells contain a diverse abundance of proteolytic activities which act in a concerted effort to maintain homeostasis. Recently, a constitutive and conserved aminopeptidase-like, proteolytic biomarker profile was identified in mammalian cells that can be harnessed for assessing viability in cell culture [26] . The assay utilizes the cellpermeant protease substrate glycyl-phenylalanyl-aminofluorocoumerin (GF-AFC) that is delivered to wells in a neutral buffer. The substrate, which lacks an amino-terminal blocking moiety (i.e. Boc, Ac, or Cbz), can be processed by aminopeptidase(s) within the cytoplasm. The AFC product released after proteolysis of the substrate is proportional to viable cell number. Viability can be measured using a fluorometer equipped with a 380-400nm excitation source and a 505nm emission filter.