Validation of specificity. Purified ALDHbr cells (20 000) were spiked into varying amounts of CBU cells (1 × 106, 3 × 106, 5 × 106, and 10 × 106). Segments were created and cryopreserved. The segments were subsequently thawed and analyzed with the potency assay, with a target of collecting 5000 ALDHbr cells. The target of 4000 to 6000 cells was met at all concentrations.