Expression of transgenes in mice, when examined with assays that can distinguish individual cells, is often found to be heterocellular, or variegated. Line‐to‐line variations in expression of a transgene may be due largely to differences in the proportion of cells in which it is expressed. Variegated silencing by centromeric heterochromatin is well described, but other factors may also affect transgene silencing in mice. Tandem arrays of transgenes themselves form heterochromatin, and some cell lineages may tend to silence transgenes because of (...) extensive facultative heterochromatin in their nuclei. The cis‐acting transcriptional control elements within a transgene inhibit silencing, and strainspecific differences in chromatin proteins may strongly influence the extent of variegation. The accessibility of multiple differentiated cell lineages in mice suggests that they may provide a tool for dissecting the role of chromatin‐mediated silencing in cell differentiation and tissue‐specific gene expression. (shrink)

Drosophila melanogaster is often considered to lack genomic 5‐methylcytosine (m5C), an opinion reinforced by two whole genome bisulfite‐sequencing studies that failed to find m5C. New evidence, however, indicates that genomic methylation is indeed present in the fly, albeit in small quantities and in unusual patterns. At embryonic stage 5, m5C occurs in short strand‐specific regions that cover ∼1% of the genome, at tissue levels suggesting a distribution restricted to a subset of nuclei. Its function is not obvious, but methylation in (...) subsets of nuclei would obscure functional associations since transcript levels and epigenetic modifications are assayed in whole embryos. Surprisingly, Mt2, the fly's only candidate DNA methyltransferase, is not necessary for the observed methylation. Full evaluation of the functions of genome methylation in Drosophila must await discovery and experimental inactivation of the DNA methyltransferase, as well as a better understanding of the pattern and developmental regulation of genomic m5C. (shrink)