We used the previously described FAST-ATAC protocol49. 10,000-20,000 cells were sorted into RPMI 1640 containing 10% fetal bovine serum. The cells were centrifuged at 500g for 5 minutes at 4C. All of the supernatant was aspirated, ensuring that the pellet was not disturbed. The pellet was then resuspended in the tagmentation reaction mix (25 uL 2X TD Buffer (Illumina, 15027866), 2.5 uL TD Enzyme (Illumina, 15038061), 0.5 uL 1% Digitonin (Promega, G9441), 22 uL H2O) and shaken at 300 RPMs at 37C for 30 minutes on an Eppendorf Thermomixer. Immediately after the incubation, samples were purified using a minElute kit (Qiagen, 28006), eluting in 10 uL. The entire sample was PCRed (a 50 uL reaction with 25 uL NEBNext, 2.5 uL F+R custom nextera primers (10 uM each; Ext. Data Table 10), 10 uL of tagmented DNA, and 12.5 uL H2O) for 5 cycles with the following program (72C, 5 minutes; 98C, 30 seconds; 5 cycles of 98C, 15 seconds, 63C, 15 seconds, 72C, 1 minute). 5 uL of the sample was qPCRed to determine the number of additional cycles required, while the rest remained on ice. The 5ul of sample was added to a qPCR mix (5 uL of PCR, 5ul of NEBNext, 0.5 uL F+R custom nextera primers, 0.09 uL of 100X SYBR (Invitrogen, S7563), 4.41 uL H2O) and qPCRed (98C, 30 seconds; 20 cycles of 98C, 15 seconds, 63C, 15 seconds, 72C, 1 minute). The number of cycles that it took to reach ⅓ the maximum fluorescence threshold in the qPCR was then applied via PCR to the original PCR sample. Libraries were cleaned using 1.5X Agencourt XP beads and ethanol washes per manufacturer's protocol. The DNA concentration of the sample was measured using Qubit and the average fragment size was determined using a TapeStation. Samples were then multiplexed and sequenced using 50 bp paired end chemistry at an average read-count of 30M reads per sample.