Maintenance of transcriptionally poised chromatin modifications states after HSC differentiation. (A-C) Changing histone modification profiles at promoters of genes showing at least 3-fold up-regulation (red bars) and down-regulation (blue bars) in MPPs (A), PreMegEs (B), and T cells (C). Up-regulated and down-regulated promoters were grouped according to whether they acquired activating (active), silencing (silent), or both activating and silencing (act/sil) modifications at each developmental stage. (D) Histone methylation and PolII profiles of promoters showing at least 3-fold up- or down-regulation of gene expression in T cells. Left panels show gene expression in HSCs, MPPs, PreMegEs, and T cells as assessed by Affymetrix. Right panels indicate the histone methylation and PolII profiles at up- and down-regulated promoters in all 4 cell types, sorted according to increasing numbers of activating histone modifications in T cells relative to HSCs.