Fifty thousand purified GC B cells were resuspended in 1ml of ATAC-seq resuspension buffer (RSB; 10 mM Tris-HCl pH 7.4, 10 mM NaCl, and 3 mM MgCl2 in water) and centrifuged at 500 r.c.f. and 4°C for 5 min. Supernatant was aspirated by pipetting and cells were resuspended in 50ul of ATAC-seq RSB containing 0.1% NP40, 0.1% Tween-20, and 0.01% digitonin and incubated on ice for 3 min. Following lysis, 1 ml of ATAC-seq RSB containing 0.1% Tween-20 was added, and nuclei were centrifuged at 500 r.c.f. and 4°C for 10 minutes. Supernatant was then aspirated by pipetting and nuclei were resuspended in 50 μl of transposition mix (25 μl 2× TD buffer, 2.5 μl tagment DNA enzyme, 16.5 μl PBS, 0.5 μl 1% digitonin, 0.5 μl 10% Tween-20, and 5 μl water) and incubated at 37 °C for 30 min in a thermomixer with shaking at 1,000 r.p.m. Reactions were cleaned up with Zymo DNA Clean and Concentrator columns. DNA libraries were prepared as described (Corces et al. Nat. Methods, 2017) and sequenced on a Hi-Seq 2000 (Illumina).