is a testing method that can be used to assess the health and purity of yeast cultures. PCR is a method of studying the genetic material, specifically the deoxyribonucleic acid (DNA), of living organisms. DNA in live cells is composed of two chains of nucleotides (building blocks) linked together to form a double helix. PCR targets specific or random regions of the DNA for analysis by amplifying the number of copies of those regions. The region of the DNA to be amplified is determined by DNA primers that bind to each side of the target sequence and trigger the amplification. The amplicon(s) can be separated according to their size in an agarose gel to generate a single band (specific PCR) or a fingerprint (random PCR). In a brewing context, fingerprints are useful to differentiate and identify yeast strains as well as to detect yeast mutations that may occur during extensive repitching of a particular culture. Specific PCR is useful to detect contaminations in wort, beer, and yeast slurries. Wild yeast or bacteria can also be targeted using species-specific primers. The appearance of a band on the agarose gel indicates the presence of the targeted contaminant, whereas the absence of a band signals that the sample is clean. Most recent technologies eliminate the need for agarose gel, thereby allowing for real-time PCR, which makes it possible to detect and quantify contaminants in as little as 3 h. This can provide brewers with critical information about the quality of potential pitching yeast, information that might previously have taken days to obtain.