The PathHunter CD28 Checkpoint Signaling cell line is a stable, engineered cell line for use in studying drug candidates (small molecules or biologics) targeting signaling checkpoints. This cell line enables assessment of ligand (e.g. CD80, CD86) based activation of CD28 receptors via detection of SH2 domain protein recruitment. This product uses EFC technology. The included cell line overexpresses PK-tagged CD28 target protein and EA-tagged SH2 domain protein. Upon activation of the receptor by its ligand or an agonist antibody, recruitment of SH2 domain protein to the phosphorylated receptor tail leads to production of EFC signal. This stable modified cell line is provided as two vials of cryopreserved cells.