The retroviral pQCXIP plasmids expressing FLAG-tagged wild type (WT) or mutants of histone H3.3 including K4M, K9M, K27M and K36 were described previously (13 (link)). The following homemade antibodies have been described: anti-MLL4#3 (14 (link)), anti-MLL3#3 (15 (link)) and anti-UTX (16 (link)). Anti-RbBP5 (A300-109A), anti-BRD4 (A301-985A100) and anti-MED1 (A300-793A) were from Bethyl Laboratories. Anti-SET1A/B antibodies were described previously (17 (link)). Anti-H3 (ab1791), anti-H3K4me1 (ab8895), anti-H3K4me2 (ab7766), anti-H3K27ac (ab4729) and H3K36me3 (ab9050) were from Abcam. Anti-MLL1N (A700-010), anti-MLL1C (A300-374A), anti-MLL2 (ABE500), anti-Pol II (17-672), anti-H3K4me3 (07-473), anti-H3K9me2 (17-648), anti-H3K27me3 (07-449) and anti-H3K36me2 (07-369) were from Millipore. Anti-FLAG-M2 (F3165) and (Z)-4-hydroxytamoxifen (4OHT) (H7904) were from Sigma. Drosophila spike-in chromatin (53083) and antibody (61686) were from Active Motif.