PathHunter® eXpress NHR Protein Interaction assays monitor the interaction of activated NHRs with Steroid Receptor Co-activator Peptides (SRCPs) using Enzyme Fragment Complementation. The cells are engineered to express two complementary fragments of β-galactosidase: the smaller ProLink™ (PK) fragment is fused to the NHR target of interest, and the larger Enzyme Acceptor (EA) is fused to the SRCP. Activation of the NHR target induces interaction of the receptor with the SRCP, forcing complementation of PK and EA. Activity of the resulting fully formed β-galactosidase enzyme is detected using a chemiluminescent substrate. The cells have been modified to prevent long term propagation and expansion using a proprietary compound that has no apparent effect on assay performance.