SP7 and Thrombocytopenia: TUNEL staining of BM samples derived from Osx-Mdm2fl/+ mice after IR showed apoptosis of hematopoietic cells, whereas the MSCs were completely devoid of TUNEL positivity, indicating that Mdm2 haploinsufficient MSCs survived the DNA damage following irradiation, and that the thrombocytopenia observed in these mice was not due to loss of MSCs (Figure 2G).