A second explanation for amplification in NTC wells is the presence of contaminating PCR products amplified in earlier qPCR experiments. This is a significant concern when the same assay is used repeatedly, and in particular when the reaction vessels are opened after the qPCR is complete. General sterile procedures can avoid this problem, such as not opening vessels after reactions or doing so in a satellite laboratory area with separate equipment and routine decontamination of laboratory space and equipment. Melt curves can be used to determine if the NTC amplification is caused by carryover contamination. If this is the case, the products in the NTC reactions will be the same species as the intended target and the melt profiles will match accordingly. If contamination is detected, then all reagents (primers, water, qPCR mix) should be replaced, equipment should be decontaminated with a 10% bleach solution, and sterile procedures should be used for future experiments. Use of 0.2 U/μL Antarctic Thermolabile UDG (NEB #M0372) can help mitigate carryover contamination for situations where it is a persistent problem or in particularly sensitive applications.