For full ChIP protocol see Borromeo et al. Development. 2014. For extraction, nuclei were liberated from cells by dounce homogenization in PBS and then fixed in 1% formaldehyde for 10 minutes at room. temperature. Fixation was terminated by adding glycine to a final concentration of 0.125M. Chromatin was sheared by using a Diagenode Bioruptor for 30 minutes on high power with 30s:30s on:off cycles. 100 μg chromatin was immunoprecipitated with 5 μg affinity-purified mouse anti-ASCL1 antibody (BD Biosciences) followed by anti-mouse Dyna beads (Invitrogen). All libraries were made according to Illumina's or NEB (NEBNEXT) ChIP-seq DNA sample prep protocol