Immunoblotting Reactivity of Anti-HER2 Monoclonal Antibodies with Recombinant Subdomains of Extracellular HER2; Crude lysates of CHO cells transfected with different HER2 extracellular subdomains were boiled for 5 min and then electrophoresed on 10% gel under reduced and non-reduced conditions along with molecular weight markers. For antibodies that recognized DI or DI+II in ELISA, immunoblotting was carried out with HER2-ECD, DI, DI+II and DII subdomains whereas mAbs which reacted with DIII or DIV by ELISA were checked using HER2-ECD, DIV, DIII+DIV and DIII subdomains under both reduced (lanes 1-4) and non-reduced (lanes 5-8) conditions. (A) 1H9, (B) 2A8, (C) 1B5, (D) 4C7, (E) Trastuzumab, (F) 1F2, (G) 2A9, (H) 1H6 and (I) 1T0 mAbs. Isolated and paired subdomains displayed ~ 25 KD and ~ 50 KD band size, respectively, whereas full HER2-ECD displayed ~ 95 KD molecular weight.