Fig 1: Calibration curves and single-cell protein quantification with microfluidic-digital proximity ligation assay (µ-dPLA). a Calibration curve for CD147 protein (inset: lower concentration region): y = 0.00172969x + 18.1043, R2 = 0.9998, dashed line indicates that limit of detection (LOD) = 2795 protein molecules. b Violin plots overlaid with scatter plots of single-cell CD147 counts in cell lines: human embryonic kidney cells (HEK293T), human lung epithelial cells (H1299), and human lung epithelial cells (A549). c Kernel density estimation of single-cell CD147 counts in the three cell lines (because kernel density estimation smooths the distribution, histograms of the single-cell data are plotted and shown in Supplementary Fig. 4). Dark purple line: HEK293T; yellow line: H1299; green line: A549. d Calibration curve for TNFR1 (inset: lower concentration region): y = 0.00151339x + 10.4555, R2 = 0.9992, dashed line indicates that LOD = 2277 protein molecules. e Violin plots overlaid with scatter plots of single-cell TNFR1 counts in cell lines: H1299 and A549. f Kernel density estimation of single-cell TNFR1 counts in the two cell lines (histograms of the single-cell data are plotted and shown in Supplementary Fig. 4). Yellow line: H1299; green line: A549. Inside the violin plots, the white dot of the box plot indicates the median of the data, the thick black bar in the center represents the interquartile range (25–75% data range) and the thin black line represents the 95% confidence interval. g On-chip calibration curve with the number of double-stranded DNA amplicons per microliter plotted against the total protein molecules in the digital PLA reaction: y = 0.00155149x + 22.3092, R2 = 0.9998, dashed line indicates the LOD, h the droplet digital PCR (ddPCR) readings for 10 HEK293T single cells are plotted with respect to LOD calculated from g shown as the dashed line, it shows all of the single-cell readings are below LOD. Error bars calculation is mentioned in the Methods section