(A) Percentages and examples of ongoing and stalled replication forks in MEFs. Inset shows examples of fibers. (B) Immunoblot analysis for Chk1 phosphorylation at S345. (C) Immunoblots of control and γ-irradiated (IR) MEF lysates probed for the indicated proteins. (D) Immunoblot analysis of asynchronously cultured MEFs transduced with control (TSIN) or Ccna2-containing (cycA2-TSIN) lentiviruses. (E) Quantification of Mre11 mRNA in polysome fractions of MEFs. (F) Cyclin A2 RNA immunoprecipitation (IP) in MEFs followed by RT-qPCR analysis for the indicated genes. IgG, immunoglobulin G; WB, Western blot. (G) As (D) but using control (TSIN) and Mre11-containing lentivirus. (H) Luciferase activity in cells transfected with 3′UTR of Mre11 mRNA. UT, untransfected; EV, empty vector. Actin was used as loading control in (B), (C), (D), and (G). Data in (A), (E), (F), and (H) are means ± SEM (N = 3 independent MEF lines per genotype). *P < 0.05, **P < 0.01 (unpaired t test).