a, Surface representation of the Dnmt3a–Dnmt3L tetramer, with two short DNA molecules adopted by superimposition of the HhaI–DNA complex structure15 onto individual Dnmt3a-C. b, Two views of the Dnmt3a–Dnmt3L tetramer with one contiguous curved DNA molecule, approximately 25 base pairs in length, covering two active sites. c, Cooperative formation of large protein–DNA complexes (red arrow) after incubation of a 146-base-pair DNA with increasing concentrations of Dnmt3a-C or Dnmt3a-C–Dnmt3L-C complex. The lower band (black arrow) corresponds to one Dnmt3L-C bound to the DNA as a result of the presence of some free Dnmt3L-C in the mixture (Supplementary Fig. 5a).