FAP-1 antigen is detected in all circulating erythrocytes. (A) Fixed and permeabilized erythrocytes were incubated with rabbit anti–FAP-1 antiserum (#B12060, Stratagene) or the same concentration of rabbit nonimmune serum and analyzed by flow cytometry. FAP-1 expression is predominately unimodal, with a small subpopulation expressing more FAP-1. (B) Erythrocyte FAP-1 antigen is the size of full-length FAP-1. Five microliters of fresh erythrocytes were boiled in 90 μL of 1× reducing loading buffer, subjected to SDS-PAGE, and immunoblotted with anti–FAP-1 antiserum¹⁷  and HRP-secondary antibody. The detected band had a mobility of 270 kDa, which corresponds to full-length FAP-1. The gel is representative of 4 experiments.