 The development of tools to manipulate the mouse genome, including knockout and transgenic technology, has revolutionized our understanding of gene function in mammals. However, these tools can lead to unintended consequences if used incorrectly. For example, when using Cree recombinase to study gene function in the male reproductive tract, researchers unexpectedly discovered that Cree expression in the brain could result in recombination in the epididymis, a tissue that takes several weeks to develop after testicular development. This suggests that Cree may be transported from its cell of origin to the epididymis via the bloodstream. Additionally, researchers have found that certain neuronal promoters and adipocyte specific promoters can also cause recombination in the epididymis, suggesting that Cree may be transported between cells by means of the circulatory system. These findings should serve as a warning to researchers who use conditional alleles, as they may not be aware of the potential for off-target recombination in other tissues. This article was authored by Vera Rinaldi, Kathleen Messamer, Kathleen Decevin, and others. We are article.tv, links in the description below.