We thank Azusa Inoue for discussions; M. Azim Surani for sharing Stella-GFP transgenic mice; Haruhiko Koseki and Miguel Vidal for sharing Ring1–/–; Ring1F/F mice; and Yao Cai and Joanna Chiu for providing Drosophila S2 cells. This work was also supported by the Open Collective Foundation Repro Grants (M.H.), the National Institutes of Health grants R35GM141085 (S.H.N.) and R21HD110146 (S.H.N. and R.M.S.). N.H. is an Investigator with the Howard Hughes Medical Institute.