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Let me give you a real life example of how PCR can 'mislead'. I work in a bioinformatics lab. We use this stuff all the time. We do protein analysis and use information from the Human Genome project and private sequence vendors. Once upon a time we got in a new upate to our sequence database and being good little employees we mined it to look for new possible drug targets. We *did* find a 'new' protein. Perplexed the hell out of us. Couldn't imagine what this particular type of protein was doing in that particular (human) tissue sample. Looked, mumbled, hypothesized, mumbled...Decided to see if this particular protein was similar to anything in any other organism genomes. (Many ongoing projects to sequence other organisms besides humans). Bingo! We got a 99.99% match (not bad considering sequencing errors are not all that uncommon in initial raw data). But it was in some sort of fish. Did a bit more poking and turns out it was a fish commonly used in Sushi. Turns out the tech doing the procedure to begin with had had sushi for lunch and hadn't washed his hands properly. (Don't even ask about e. coli contaminants in the human genome database data...although they're screened out these days because we compare proteins to other organisms that are sequenced & commonly appear in our 'environment' as it were...just didn't do the compare to sushi fish...)