In this pivotal study, the authors demonstrated that targeted deletion of Dicer1 -an RNAse III endonuclease involved in miRNA biogenesis- from murine osterix-expressing osteoprogenitors, but not terminally differentiated osteoblasts, resulted in defective osteoblast differentiation and in the initiation and propagation of a form of myelodysplasia associated with leukopenia and lymphocytopenia. This evidence concerns the gene DICER1 and Decreased total leukocyte count.