Core histone gene expression in HeLa S3 cells has been examined as a function of the cell cycle using cloned human histone gene probes. Total cellular histone mRNAs were analyzed by Northern blot analysis, and their relative abundance shown to be temporally coupled to DNA synthesis rates in S phase. The in vivo incorporation of 3H-uridine into at least fifteen heterologous histone mRNAs (in one hour pulse intervals at various times in the cell cycle), was monitored by hybrid selection. Hybridized RNAs were eluted and resolved electrophoretically to give both a quantitative and qualitative assay for multiple mRNA species. Maximal incorporation of 3H-uridine into histone mRNAs precedes their maximal accumulation, indicating that transcriptional regulation is predominant in early S phase. The turnover of histone mRNAs in late S occurs in the presence of a reduced apparent transcription rate, indicating that post-transcriptional regulation is predominant in late S. All the detected multiple histone mRNAs are coordinately regulated during the HeLa cell cycle.

Mutations of the human B-RAF gene are detected in f8% of cancer samples, primarily in cutaneous melanomas (70%). The most common mutation (90%) is a valine-to-glutamic acid mutation at residue 600 (V600E; formerly V599E according to previous nomenclature). Using a Cre/Lox approach, we have generated a conditional knock-in allele of V600E B-raf in mice. We show that widespread expression of V600E B-Raf cannot be tolerated in embryonic development, with embryos dying f7.5 dpc. Directed expression of mutant V600E B-Raf to somatic tissues using the IFN-inducible Mx1-Cre mouse strain induces a proliferative disorder and bone marrow failure with evidence of nonlymphoid neoplasia of the histiocytic type leading to death within 4 weeks of age. However, expression of mutant B-Raf does not alter the proliferation profile of all somatic tissues. In primary mouse embryonic fibroblasts, expression of endogenous V600E B-Raf induces morphologic transformation, increased cell proliferation, and loss of contact inhibition. Thus, V600E

Mutation rates at two expanded simple tandem repeat loci were studied in the germ line of first-and second-generation offspring of inbred male CBA͞H, C57BL͞6, and BALB͞c mice exposed to either high linear energy transfer fission neutrons or low linear energy transfer x-rays. Paternal CBA͞H exposure to either x-rays or fission neutrons resulted in increased mutation rates in the germ line of two subsequent generations. Comparable transgenerational effects were observed also in neutron-irradiated C57BL͞6 and xirradiated BALB͞c mice. The levels of spontaneous mutation rates and radiation-induced transgenerational instability varied between strains (BALB͞c>CBA͞H>C57BL͞6). Pre-and postmeiotic paternal exposure resulted in similar increases in mutation rate in the germ line of both generations of CBA͞H mice, which together with our previous results suggests that radiation-induced expanded simple tandem repeat instability is manifested in diploid cells after fertilization. The remarkable finding that radiationinduced germ-line instability persists for at least two generations raises important issues of risk evaluation in humans.