Shares of Protagonist Therapeutics Inc. (NASDAQ:PTGX) fell 62% to $17.53 on Sept. 17 after it reported finding malignant skin tumors in a mouse model test of its most advanced candidate, rusfertide for blood disorders. After the company notified the FDA, the regulator put the program on a clinical hold, leading dosing of patients in all ongoing trials of rusfertide to be halted for now. The development could impact Protagonist's ability to start phase III testing of the candidate in polycythemia vera (PV) early in 2022, as well as efforts to expand its development to a third indication beyond PV and hereditary hemochromatosis (HH) by the end of this year, as it has planned.