Adhesion of CD34⁺ cells to immobilized Opn is specifically mediated by β₁ integrin. Calcein-AM–labeled BM CD34⁺ cells were assayed for their ability to attach to GST fusion–tagged full-length human Opn (▪). The ability of β₁ integrin to mediate this process was analyzed by preincubating the cells in the specific β₁ integrin–blocking antibody P5D2 prior to being deposited in the wells (□). Data are the mean ± SEM of triplicate wells representing 3 independent experiments.