DIfferent strains of mice have different resistance to retrovirus infection. Some strains are highly resistant, others quite susceptible. At least some of this difference in susceptibility comes down to different expression levels of mouse APOBEC3: High expression of the gene gives good resistance to some retroviruses, low expression gives less resistance.
How come some strains have higher expression than others? Turns out that it’s because a retrovirus inserted in the APOBEC3 region of the genome of certain mouse strains, and that insertion cranks up expression of the APOBEC3.
We discovered that the mA3 allele in virus resistant mice is disrupted by insertion of the regulatory sequences of a mouse leukemia virus, and this insertion is associated with enhanced mA3 expression. ((Sanville, B., Dolan, M., Wollenberg, K., Yan, Y., Martin, C., Yeung, M., Strebel, K., Buckler-White, A., & Kozak, C. (2010). Adaptive Evolution of Mus Apobec3 Includes Retroviral Insertion and Positive Selection at Two Clusters of Residues Flanking the Substrate Groove PLoS Pathogens, 6 (7) DOI: 10.1371/journal.ppat.1000974))
So perhaps low APOBEC expression allowed retrovirus infection, which led to insertion of the retrovirus genome, which increased APOBEC3 expression and provided resistance to further retrovirus infection.