Reduced primary cell proliferation and premature senescence in Dicer-ablated primary MEFs. (A) FACS indicates fewer Dicer-ablated cells undergo DNA replication. (B) Proliferation assay of Dicer-wt and Dicer-ablated cells reveals that Dicer loss inhibits cell proliferation. (C) Proliferation assay of Dicer-heterozygous and Dicer-ablated cells after induction of Cre activity. (D) SA-βGal staining of Dicer-wt or Dicer-ablated cells at day 12. Bar, 200 μM. (E) SA-βGal staining of Dicer^c/c MEFs after Ad-Cre or Ad-control infection. (F) DAPI-staining of DNA reveals dramatically altered heterochromatin in Dicer-ablated cells (right). Bar, 10 μM.