RNase H2–deficient cells show impaired proliferation and accumulate in G2/M. (A) ³H-thymidine incorporation by E14.5 Rnaseh2b^KOF/KOF (KOF) versus Rnaseh2b^KOF/WT or RnaseH2b^WT/WT control (Ctrl) fibroblasts. (B) DNA content analysis. Fibroblasts from E14.5 Rnaseh2b^KOF/KOF (n = 10) or control (WT or heterozygous, n = 56) embryos as well as fibroblasts from E18.5 Rnaseh2b^KOF/KOF (n = 2) or control (WT or heterozygous, n = 5) embryos were analyzed 1 or 4 wk after initiation of the cultures. One representative result is displayed. (C) Lethally irradiated recipients were transplanted with 4 × 10⁶ fetal liver cells from E14.5 Rnaseh2b^KOF/KOF (n = 3) or WT embryos (n = 3). Chimerism was quantified by detection of the congenic CD45.1/2 marker on CD11c⁺Gr1⁺ neutrophilic granulocytes. (D) RNase H2 activity in sorted splenic B220⁺CD19⁺ B cells from Rnaseh2b^FLOX/FLOX CD19-Cre⁺ mice (n = 2) and controls (WT/FLOX Cre⁺, n = 1; and WT/FLOX Cre-negative, n = 2). Purity of the sorted B cells was 96.5–97.7%. (E) Numbers of splenic CD19⁺B220⁺ B cells in adult Rnaseh2b^FLOX/FLOX CD19-Cre⁺ (KO, n = 6) and controls (Ctrl, FLOX/FLOX Cre⁻ or FLOX/WT CRE⁺, n = 6). (F) ³H-thymidine incorporation by Rnaseh2b^FLOX/FLOX CD19-Cre⁺ (n = 9) and control (Ctrl, FLOX/FLOX Cre⁻ or FLOX/WT CRE⁺, n = 9) B cells. Total splenocytes were analyzed after 3 d of culture in the presence of LPS. *, P < 0.05 (two-tailed Student’s t test). (G) DNA content analysis. Splenic B cells from Rnaseh2b^FLOX/FLOX CD19-Cre⁺ (Δ/Δ, n = 6) and control mice (Ctrl, FLOX/FLOX Cre⁻ or FLOX/WT CRE⁺, n = 6) were analyzed after 3 d in vitro LPS stimulation. One representative result is displayed. Error bars indicate ±SD in all graphs.