Merck & Co. Inc.’s deal to pay $23 in cash for Harpoon Therapeutics, bringing the equity value to about $680 million, further invigorated the delta-like ligand 3 (DLL3) development zone, where Amgen Inc. enjoys a lead with its prospect tarlatamab in the same class. DLL3 is an inhibitor ligand of the Notch pathway associated with tumorigenesis.