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Unfortunately even with good laboratory practices the occasional contamination may happen. In order to evaluate action to take regarding occasional contamination, it is important to establish the type of contamination and their origin(s). Sources of contamination and ways of spreading can be very broad, varying from laboratory personnel to unfiltered air to equipment used. All reagents, incubators, and equipment are possible causes that need to be investigated. There can also be instances of air quality in the building causing issues. I invite you to refer to table present in document at: CLS-CGTS-077 https://www.corning.com/catalog/cls/documents/application-notes/CLS-CG-TS-077.pdf. The table gives a first impression on type of contaminations, possible sources, and troubleshooting approaches.