Chem-1 Parental Control Frozen Cells are designed for use as a negative control alongside our GPCR-expressing Ready-to-Assay Frozen Cells in the Chem-1 background. Chem-1 cells lack endogenous expression of most GPCRs, although they endogenously express trypsin and thrombin receptors that may be used to confirm responsiveness of the cells in calcium assays.