CSF1R and Splenomegaly: In support of this idea, we did not detect significant numbers of CSF-1R+ ECFP− cells in the spleen (data not shown), suggesting that the expanded myeloid ECFP− cells in the spleen did not express Slfn4. In summary, a threshold level of ectopic Slfn4 expression appears to generate a form of myelodysplasia, because the phenotype was not observed in lines expressing lower levels of Slfn4. Given that the splenomegaly phenotype was apparent in only one line however, we cannot completely exclude the possibility that insertional effects generate this phenotype.