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Transgenic animals, created by altering an organism’s genetic information through recombinant DNA technology, are invaluable in many facets of scientific research; they enable us to investigate genetic disorders, search for possible therapies and help us develop new drugs. Recently, however, Evann Corbo-Rodgers and colleagues from the Perelman School of Medicine at the University of Pennsylvania encountered a spanner in the works when investigating transgenic mice, as described in their recent correspondence published in Nature Immunology, and the whole story begins with a little parasite.

Fur mites are a very common problem in rodents – lab mice being no exception – and ivermectin, an anti-helmintic drug used against a broad range of parasites, is the preferred treatment. In Corbo-Rodgers et al.’s study, highlighted by F1000 members Virginia Papaioannou and Ripla Arora of the Developmental Biology Faculty, the lab mice undergoing tamoxifen experimentation had been treated for mites with ivermectin. The authors were perplexed to observe genetic changes in the mice – alleles disappearing. What they didn’t know was that these were a result of ivermectin treatment rather than the experimental tamoxifen. Furthermore, the offspring of ivermectin-treated mice also exhibit these changes, even those born up to 15 weeks after the mother’s treatment.

These changes are evident because of the route by which the transgenic animals were generated: the Cre/loxP system. It allows us to selectively modify an organism’s DNA so that it expresses a particular sequence in a particular tissue. Cre recombinase is an enzyme that catalyses site-specific recombination events according to the position of loxP on a DNA strand, where only the information between two LoxP markers is targeted by the enzyme. Tamoxifen, an estrogen receptor antagonist commonly used as a treatment for breast cancer, allows the Cre recombinase entry into specific cells’ nuclei, allowing the recombination to take place, making it a desirable accomplice in transgenic research.

While these findings could have an impact on the validity of experimental results, they could also be the red flag we need – what if this glitch occurs in other conditions, in different tissues, under different conditions? How do we know that this hasn’t happened before? “Even with this uncertainty,” conclude Papaioannou and Arora, “what is abundantly clear is that standard procedures for dealing with pinworm and fur mites can cause serious disruptions to research using floxed alleles. Learning about this phenomenon may result in more than one “Ah ha!” moment by explaining previously mysterious excision of conditional alleles.”

Read the full F1000 Recommendation here.