SPR-based analysis of homophilic CEACAM1 ectodomain interactions. Sensorgrams were recorded in a BIAcore 2000 instrument. Rat and human CEACAM1 D(1–4)-Fc proteins were immobilized at a level of 7,100 response units (RU). (A and B) Five concentrations of rat D(1–4)-His (0.36–1.79 g/l, corresponding to 4.4–22.2 µM monomer) were analyzed in HBS/P20/3 mM EDTA (A) or HBS/P20/2 mM Ca²⁺/2 mM Mg²⁺ (B). The corrected responses (cRU) for binding to rat D(1–4)-Fc, obtained by subtracting the response in the reference lane (human D(1–4)-Fc), are shown as black data points. The results of global curve fitting to each of the three reaction models (models 1–3; see Results, Materials and methods, and Fig. S1) are shown as residual plots above the sensorgrams. The curve fittings shown in the sensorgrams are according to model 1 (A, purple curves) and model 3 (B, red curves).