The domesticated transposon protein L1TD1 associates with its ancestor L1 ORF1p to promote LINE-1 retrotransposition. - Kavaklioglu G; Podhornik A; Vcelkova T; Marjanovic J; Beck MA; Phan-Canh T; Mair T; Miccolo C; Drino A; Doni M; Egger G; Chiocca S; et al. (ELife, 2025)