In mice, a new study reveals that pups that receive less maternal care have more repeated genetic sequences, called transposons, in neurons that reside in their hippocampus. Of note, the implications of this phenomenon are unknown, meaning further research is needed. An enduring question in science is the influence of and intersection between nature versus nurture. Indeed, evidence suggests that the amount of licking and grooming from a mother mouse to her pup can alter the pup's gene expression. Here, Tracy Bedrosian and colleagues sought to explore the influence of maternal care on the prevalence of pups' transposons, sequences that can spontaneously replicate within the genome. In particular, they studied L1 retrotransposons, of which the mouse genome retains more than 3,000. Mother mice and their newborn pups were monitored for two weeks, then assigned to two different groups based on low and high levels of maternal care. Pups that received less maternal care exhibited noticeably more transposon replication in the hippocampus, the authors report. Intriguingly, this effect was not observed in the frontal cortex or heart, hinting that it is unlikely to be a result of inherited differences in the numbers of L1 copies. As well, experiments where pups were raised by foster mothers support the influence of maternal care, rather than genetic makeup. Further investigation reveals that methylation of transposons may, in part, drive this phenomenon. Lastly, Bedrosian et al. point to a recent report that childhood stress and adversity result in hypomethylation of retrotransposons in humans, noting however that the average human genome has many fewer (about 100) active copies than the mouse genome with its 3,000.