Absence of the Δ319 G-CSFR at the cell surface. Surface proteins from CHO cells transfected with only the WT or the Δ319 G-CSFR, or both receptor forms were left unlabeled or labeled with biotin (+ Biotin) using the membrane impermeable sulfo-NHS-LC-Biotin (Pierce), lysed, then immunoprecipitated with anti-Myc and/or anti-V5 antibodies, and immunoblotted with either anti–G-CSFR (sc9173) to detect the WT and Δ319 receptor forms (A) or with HRP-conjugated streptavidin to detect biotinylated surface proteins (B).