Loss of H3f3b causes a semilethal phenotype and in a subset of embryos strongly impairs overall growth. (A) Ratios of animals of the indicated ages from heterozygous intercrosses indicate substantial lethality among mature knockout (KO) animals.* 0.01 < P <0.05 by Chi Square. (B, C) KO E12.5 embryos exhibit strong reductions in overall growth. Scale bar = 1 mm. (D) Wildtype (WT) 63 and KO 56 E12.5 mouse embryonic fibroblasts (MEFs) stained with anti-H3.3 antibodies show higher levels of H3.3 in WT MEFs. (E) Chromosomal bridges were observed much more frequently in KO nuclei than in WT nuclei. DAPI staining of a KO 56 nuclei shows representative example of a KO chromosomal bridge. (F) H3f3b KO MEFs exhibit a statistically significant 1.45 to 1.5-fold increase in DAPI mean fluorescence intensity (MFI) per nuclei, and a 1.21 to 1.36-fold increase in the amount of DAPI MFI per unit of nuclear area. (G) H3f3b KO MEFs display a significant 1.30 to 1.5-fold increase in the number of pericentric heterochromatic DAPI foci per nuclei, and a significant increase in the number of DAPI-positive foci per unit of nuclear area. Scale bars = 10 um.