Chromatin signature

Allele-specific gene expression is an integral component of cellular programming and development and contributes to the diversity of cellular phenotypes [1, 2]. Allelic differences in gene expression are mediated by either parent-of-origin-specific selection (imprinting) or stochastic selection of alleles for activation and/or silencing. The importance of genomic imprinting has recently been highlighted by RNA sequencing studies that demonstrated widespread allelic differences in gene expression in mouse brain affecting more than 1,300 genes [3]. The extent of sex-and stagespecific expression of individual alleles emphasizes the essential role of allelic transcriptional regulation in development. In addition to the extensive occurrence of imprinted parent-of-origin-specific expression, gene expression patterns of clonal cell populations are also modified by random or stochastic silencing of either the maternal or paternal allele. Wellknown loci displaying allele-specific expression include odorant receptor genes, immunoglobulins and various receptor proteins [4-6]. Additionally, previous large-scale studies have provided new data demonstrating that parent-of-origin-specific expression is employed much more frequently than previously thought [7]. These new findings illustrate the scale and complexity of genomic allele-specific expression. However, the precise molecular mechanism underlying the allelic bias in gene expression is not very well understood.