Drosha deficiency abrogates miRNA but not rRNA processing. (A) CD4-cre efficiently deletes exon 9 from the conditional Drosha transcript from the DP thymocyte stage onwards. cDNA from the indicated populations was analyzed for expression of the Drosha exons 5/6 or exon 9/10 junction by quantitative RT-PCR. DN4, CD90⁺TCRβ^loCD4/8/44/25⁻; DP, CD4⁺8⁺; CD4SP, TCRβ^hiCD24^lo4⁺8⁻; CD8SP, TCRβ^hiCD24^lo4⁻8⁺. (B) Drosha protein expression in DP thymocytes, and peripheral TCRβ⁺ T cells and B220⁺ B cells from Drosha^F/Δ CD4-cre and control Drosha^F/+ CD4-cre mice. Molecular weights are shown. (C) Total RNA from DP thymocytes and peripheral T and B cells was resolved on a polyacrylamide gel and Northern blotted for expression of mature miRNAs. Also shown is the ethidium bromide gel for expression of small rRNA and transfer RNA species. (D) Total RNA was resolved on an agarose-formaldehyde gel and Northern blotted for expression of pri-miR-150. Also shown is the ethidium bromide gel for expression of the large rRNA subunits.