Known unknowns for allele-specific expression and genomic imprinting effects.

Recent studies have provided evidence for non-canonical imprinting effects that are associated with allele-specific expression biases at the tissue level in mice. These imprinting effects have features that are distinct from canonical imprinting effects that involve allele silencing. Here, I discuss some of the evidence for non-canonical imprinting effects in the context of random X-inactivation and epigenetic allele-specific expression effects on the autosomes. I propose several mechanisms that may underlie non-canonical imprinting effects and outline future directions and approaches to study these effects at the cellular level in vivo. The growing evidence for complex allele-specific expression effects that are cell- and developmental stage-specific has opened a new frontier for study. Currently, the function of these effects and the underlying regulatory mechanisms are largely unknown.