Linearity of Laser-amplified Qdot-RPPA for quantifying protein levels. A. Human H358 (p53 null) lung cancer cell lysate was mixed with H2009 (p53 mutant) cell lysate proportionally (%) as indicated, then printed with seven serial dilutions in triplicate on protein array. The array was probed for total p53 levels and signals amplified by a confocal laser Qdot scanner. B. The same lysates in A., but signals amplified by CSA kit (Dako). C. Linear regression curve of 1:8 dilution from Panel A is shown here. X axis is % of p53 mutant lysate in the mixed sample and Y axis is the corresponding Qdot intensity corrected with total protein loading, then subtracted the background signal at the 0% of p53. Black squares is the data from Panel A; red diamond is the data from Panel B. 25% change of p53 can be detected by Laser-amplified Qdot-RPPA as indicated by the black linear regression line. In contrast, linearity was lost with HRP-amplification shown as a red sigmoid curve.