Western blot was performed using Human IgG kappa Gatipotuzumab Recombinant Human Monoclonal Antibody (Product # MA5-42157) (Fig. a) and Human IgG lambda Anetumab Recombinant Human Monoclonal Antibody (Product # MA5-42007) (Fig. b). Whole cell lysate (30 µg lysate) of T-47D (Fig. a Lane 1), MCF7 (Fig. a Lane 2), NIH:OVCAR-3 (Fig. b Lane 1) and PC-3 (Fig. b Lane 2) were electrophoresed using NuPAGE™ 3-8% Tris-Acetate Protein Gel (Product # EA0378BOX) and NuPAGE™ 4-12% Bis-Tris Protein Gel (Product # NP0322BOX) respectively. Resolved proteins were transferred onto a nitrocellulose membrane (Product # IB23001) by iBlot® 2 Dry Blotting System (Product # IB21001). Blots were probed with the primary antibody (1:2000 dilution) and Mouse anti-Human Kappa Light Chain Secondary Antibody (Product # MA1-10386, 1 µg/mL). Chemiluminescent detection was performed using SuperSignal™ West Atto Ultimate Sensitivity Substrate (Product # A38554) and detected by chemiluminescence with Goat anti-Mouse IgG (H+L), Superclonal™ Recombinant Secondary Antibody, HRP (Product # A28177, 1:20,000 dilution). Bands at 260 kDa corresponding to Gatipotuzumab (MUC1) was observed in T-47D as compared to MCF7 which is a low expressor (Fig. a). No band corresponding to Anetumab (MSLN) was observed in NIH:OVCAR-3 (Fig. b) implying that Mouse anti-Human Kappa Light Chain Secondary Antibody is specific to the Human kappa light chain.