HEXIM1 OE regulates erythroid cell cycle progression by promoting both RNAPII pausing and recruitment. (A) Heat map of paused genes in EV, HEXIM1 OE, and Y271A OE cell lines; values represent z score of the average PI in the corresponding cell line; unsupervised k-means clustering was performed to acquire clusters of genes with different pausing status in each line. (B) Enriched pathways for the clusters of genes with a higher PI in HEXIM1 OE cell lines. (C) PI of gene sets cell cycle arrest genes” and cell cycle checkpoint genes. (D) Doubling time of EV, HEXIM1 OE, and Y2A OE HUDEP-2 cells. (E) Scheme of key regulators of S-phase entry. (F) Changes of chromatin accessibility, HEXIM1, and RNAPII occupancy at CCNE2 via ATAC sequencing and CUT&RUN. (G-H) Representative western blot (G) and quantification (H) of key regulators of G1/S phase progression; additional regulators are shown in supplemental Figure 10E. (I) Cell cycle analyses in HUDEP-2 and CD36⁺ selected primary erythroblasts via 5-bromo-2-deoxyuridine staining. n = minimum of 3 replicates; ∗P < .05; ∗∗P < .005.