SP7 and myeloproliferative disorder: We first used the MPLW515L retroviral model of MPN in which c-Kit+ cells from WT mice transduced with retrovirus expressing MPLW515L and GFP are transplanted into congenic Tgfbr2fl/fl or Osx-Cre Tgfbr2fl/– mice; we also included Tgfbr2fl/– mice to control for global Tgfbr2 haploinsufficiency (Figure 1B).