CCCTC binding factor (CTCF) regulates nucleolar number. (A) Efficient deletion of CTCF in mouse embryonic fibroblasts (MEFs). MEFs carrying the conditional Ctcffknockout allele were infected (+) or not (-) with a lentivirus expressing Cre recombinase. After 4 days, cell extracts were analyzed for residual CTCF. Mta2 was used as loading control. In MEFs, Ctcf deletion with lentiviral Cre was very efficient, with > 90% of cells infected. (B, C) Distribution of upstream binding factor (UBF) in MEFs. Primary MEFs carrying the conditional Ctcffknockout allele were either infected (+CrE) or not (-CrE) with a lentivirus expressing Cre recombinase. Cells were fixed, and incubated with antibodies against UBF (because both the CTCF and UBF antibodies used for immunofluorescence analysis were mouse monoclonals, we could not perform a combined CTCF/UBF stain. (B) Single image; (C) immunofluorescence results from several images were quantified.