A Master Cell Bank (MCB) is produced from an original cell line seed stock clone. These cells are cryopreserved in multiple vials in order to prevent genetic variation and potential contamination by reducing the total number of times a cell line is passaged or handled. MCBs contain a set number of cells per vial, have been cultured under well-defined conditions and are stored in a stable state at ultra-low temperatures. Cells from an MCB are expanded to a working cell bank (WCB), from which assay-ready cells are derived. This two-tiered cell banking system, when prepared correctly and managed with appropriate documentation and testing, can provide a reproducible and dependable supply of cell-based assays for many years.