To assess whether Hiwi merely associates with inhibition of differentiation and tumorigenesis, or whether Hiwi may directly inhibit differentiation and promote sarcomagenesis, we expressed Hiwi in sarcoma precursors (i.e., mesenchymal stem cells, MSCs; Figure 1B; Figure S2 and Supporting Information S1). The gene discussed is PIWIL1; the disease is sarcoma.