WDR5 and leukemia: Specifically, WDR5 and RBBP5 of the mixed-lineage leukemia histone methylation complex catalyze histone H3 methylation on lysine K4 (H3K4) that occupies transcription start sites (Wysocka et al, 2005) and hydroxyurea (HU)-stalled replication forks (Ray Chaudhuri et al, 2016).