Experimental design of parabiosis studies. c-mpl^−/− and WT female 10- to 14-week-old mice were linked in parabiosis by suturing tissue from shoulder to hip. After 10 days, approximately 50% of the blood granulocytes and lymphocytes in each parabiont are of partner origin, demonstrating that the circulations are joined (data not shown). After 4 to 6 weeks, the parabionts are separated and the percentages of partner phenotype granulocytes in marrow and blood are assessed by flow cytometry for CD45.1 (PeP3^b) and CD45.2 (c-mpl^−/−; Experiment 1) or β-galactosidase activity (ROSA26; Experiment 2). The transfer of HSCs from the WT parabiont to the c-mpl^−/− parabiont was quantitated by the transplantation of marrow cells into irradiated male tpo^−/− recipients. The contribution WT HSCs to recipient hematopoiesis was assessed 3 months later. In each experiment, we proved that there was no recovery of endogenous tpo^−/− marrow cells using Southern-blot analyses and probing for the Y-chromosome (data not shown). Methodologic details can be found in Abkowitz et al¹⁵  and Chen et al.¹⁶