a, Domain architecture of CTCF. CTCF fragments tested for SA2–SCC1 binding by GST pulldown analysis are indicated. The region that retains SA2–SCC1 is highlighted in magenta. b, Summary data showing results of GST pulldowns. The input and the bound fractions were analysed by SDS–PAGE. CTCF fragments that bind SA2–SCC1 are shown in magenta. The experiment was repeated once. c, ITC curves. The binding stoichiometry (N) and dissociation constants (Kd) are indicated. The experiment was repeated three times, with consistent results. d, Fo − Fc omit electron-density Fourier map contoured at 3σ. e, LIGPLOT representation of the interaction between the CTCF peptide and SA2–SCC1. The CTCF peptide is shown in magenta, SA2 in blue and SCC1 in green bonds.