ERBB2 and cancer: The genetically fused 4D5scFv-PE40, containing single HER2-specific trastuzumab variant 4D5scFv as a targeting module, killed HER2-positive cancer cells with IC50 value as low as 10–20 pM [22]; at the same time, the IC50 value for trastuzumab-PE40 chemical conjugate was about 100 nM [23], although the affinity of trastuzumab alone is higher than that of 4D5scFv: the estimated KD are 1.8 nM and 5.2 nM, respectively [24,25].