0.05-0.5x106 cells were snap-frozen on dry ice for 15 minutes after 3 washes with PBS. Chromatin was prepared and ultra-low-input ChIP performed according to Brind'Amour et al., 2015 (Nature Communications). ChIP DNA was end-repaired, A-tailed, and ligated to Illumina PE adapters. Ligated fragments were PCR amplified using Illumina indexed primers for 12 cycles. DNA was purified using Ampure XP DNA purification beads between each step. Full methodology is available in Brind'Amour et al., 2015 (Nature Communications).