Overview of droplet digital PCR. The RainDrop ddPCR workflow consisted of three steps: (1) PCR reaction mixtures for each sample were pipetted into one of eight wells on the Source microfluidic chip, which was next placed above empty PCR tubes in the RainDrop Source. Pressure-driven air flow forced the aqueous mixture into the chip along with a surfactant-containing fluorocarbon oil to generate five picoliter droplets, and then deposited automatically into the PCR tube strip. (2) The PCR tube strip was placed into a standard thermal cycler for endpoint PCR amplification, with single-target-molecule-containing droplets resulting in specific probe hydrolysis (PCR+) and bright fluorescence and the majority of droplets, containing no target molecule, resulting in only background probe fluorescence (PCR−). (3) Each droplet’s fluorescence was detected using a Sense microfluidic chip. The fluorescence was detected and processed into a two-dimensional scatter plot display, custom software was used to draw appropriate gates for each droplet endpoint cluster, and the number of droplets within each gate was counted.