PathHunter Internalization Assays are cell-based assays that measure internalization of single-pass membrane receptors, e.g. receptor tyrosine kinases (ErbB-2), immune checkpoint receptors (CD33), mediated by small molecule or biologic therapeutics. Measuring drug-mediated receptor internalization can provide insight into diseases, drug tolerance, unwanted side effects and helps in identifying safer drugs. These receptor internalization assays are valuable tools for therapeutics that require internalization, which is used as a key determinant for their efficacy, such as for antibody-drug conjugates (ADCs). These assays utilize stable cell lines and a simple, homogenous protocol and can easily be implemented in a high-throughput format.