Fig 1: Detection of secreted and membrane proteins in blood by single-molecule imaging.(A) Schematic diagram of secreted PSA release from a tumor cell (lime) into a blood vessel (red). SMAC images (B) and shape analysis (C) of purified human PSA at femtomolar concentrations in aqueous buffer. (D) Quantification of PSA in lysate from different numbers of human prostate cancer cells (LnCaP) added into aqueous buffer. SMAC images (E) and quantification of PSA (F) in lysate from one LnCaP cell in either aqueous buffer or human plasma. (G) PSA levels in the blood of patients with prostate cancer (n = 5) and healthy male (n = 4) and female (n = 4) control blood donors. (H) Schematic diagram of membrane-bound programmed death-ligand 1 (PD-L1) release from a tumor cell (lime) into a blood vessel (red). SMAC images (I) and shape analysis (J) of purified human PD-L1 at femtomolar concentrations in aqueous buffer. (K) Quantification of circulating PD-L1 levels in patients with high-grade squamous intraepithelial lesions (HSILs; n = 6) and healthy donors (n = 5). PSA data and PD-L1 data are expressed as means ± SD. Scale bars, 4 µm.