HEXIM1 OE promotes erythroid expansion. (A) HEXIM1 and 7SK levels in HUDEP2 cultures with OE of HEXIM1. (B) Live fold expansion HEXIM1 OE and EV control cultures in expansion conditions. (C) Quantified doubling time for HEXIM1 OE compared with empty vector. (D) Cytosopins of HEXIM1 OE and EV cultures. (E) Live fold expansion HEXIM1 OE and EV control cultures in maturation conditions. (F) Quantification of CD235a levels in HEXIM1 OE and EV control cells on 4 days of maturation (i). Representative images from IDEAS software showing brightfield (BF), DNA (DRAQ5 staining), and CD235a (GYPA) staining (ii). (G) Cytopsins of HEXIM1 OE and EV cultures on day 6 and day 10 of maturation. (H) Expression of HEXIM1 in clonal lines of HUDEP2 cells expressing EV, WT HEXIM1, or HEXIM1 containing 2 amino acid substitutions: Y271F and Y274F (YYFF). (I) Live fold expansion of clonal lines expressing EV, WT HEXIM1, or YYFF HEXIM1 in expansion conditions. (J) Live fold expansion for indicated lines in maturation conditions. (K) CD235a levels in indicated populations. (L) Imaging flow cytometric analyses of HEXIM1 OE and EV cells following HEXIM1 OE on day 3 of the CD34⁺ erythroid culture shown in Figure 1A. (i) Quantification of cell size, nuclear size, and CD235a expression. (ii) Representative images from Ideas software. *P < .05 compared with empty vector control line.