Ott1 regulates the ratio between full-length and truncated c-Mpl isoforms. (A) Relative c-Mpl expression determined through qPCR of total c-Mpl RNA between wild-type (WT) and Ott1 KO cells. Total RNA isolated from flow-sorted LT-HSC (Lin^–c-Kit⁺Sca1⁺Cd34^–), ST-HSC (Lin^–c-Kit⁺Sca1⁺Cd34⁺) (WT, n = 4; Ott1 KO, n = 3), and Lin^–c-Kit⁺Sca1^–CD34^–FcγR^– (MEP, n = 4) from pIpC-treated Ott1 WT or Ott1^flox/nullMx1-cre (Ott1 KO) adult bone marrow and isolated megakaryocytes (n = 3) from Ott1 WT or Ott1^flox/nullSox2-cre E14.5 fetal liver cultures. Error bars represent 1 standard deviation. (B) Schematic of c-Mpl pre-mRNA splicing for Mpl-FL (solid line) and Mpl-TR (dotted line). (C) Diagram of predicted proteins generated from Mpl-FL and Mpl-TR isoforms. TM, transmembrane domain; dotted line, plasma membrane. (D) Ratio of Mpl-TR to Mpl-FL isoforms from qPCR of total RNA isolated in (A). (E) Western blot of lysates from E14.5 fetal liver megakaryocyte cultures of Ott1 WT and Ott1 KO (Ott1^flox/nullSox2-cre) probed with anti-c-Mpl and anti-GAPDH. Performed in triplicate. (F) Ratio of MPL-TR (gray) to MPL-FL (white) in HSC-enriched (CD34⁺) and megakaryocyte-enriched (CD34^–CD41⁺) human bone marrow populations (n = 5).