Conversely, the loss of signal-induced proliferation-associated gene 1 (Sipa1), a RAP1 GTPase-activating protein expressed mainly by mesenchymal stem/progenitor cells and endothelial cells induced significant alterations in the BM niche prior to the initiation of MDS/MPN in mice [72]. The gene discussed is SIPA1; the disease is myelodysplastic syndrome.