The 50% inhibitory concentration (IC50)of the 4D5scFv-PE40 immunotoxin for HER2-overexpressing cancer cells was2.5–3 orders of magnitude lower compared to that for CHO cells notexpressing this tumor marker and was 2.5–3 orders of magnitude lower thanIC50 of free PE40 for HER2-overexpressing cancer cells. The gene discussed is ERBB2; the disease is cancer.