Digital PCR detects single molecule amplifications. This is done by splitting the sample into micro-droplets in which single target reactions are monitored. A labeled probe incorporated into each amplification product allows the reactions to be detected and monitored. The number of droplets with amplification is counted to give the total target number of the sample. Small differences in copy numbers can be determined because quantitation is performed on a linear scale. This differs from conventional PCR where amplification is detected on a log scale and small differences target numbers can’t be differentiated. The ability to detect small differences in target number between samples makes this technique valuable in the detection of single nucleotide polymorphisms, gene expression and copy number variations.