This review has surveyed the current literature on random and imprinted monoallelic expression from the unusual perspective of considering that these two forms of monoallelic expression (MAE) may be regulated by similar mechanisms. The true extent of genes which show MAE is not yet known. Imprinted MAE is thought to involve 0.1-0.2% of mammalian genes, while the number of genes in the random MAE category cannot at present be predicted since very few genes have been examined with assays that distinguish parental transcripts in single cells. The perspective adopted in this review is speculative since only one gene (XIST) is known which displays both random and imprinted MAE. However, a recent report that H19 may also show random allelic expression, suggests that MAE should be further investigated as a gene regulation mechanism in mammals.

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