I was looking around to try to find a good short explanation of what allelic imbalance is, but was unable to find one. I eventually figured it out, and wanted to make a post to clarify this for future searchers:

What is allelic imbalance?

A difference in the expression between two alleles. Humans are diploid organisms, which means we have 2 copies of each gene. Normally, these two copies are expressed at the same level. This means that the mRNA transcript from the mother and the transcript from the father will have roughly the same number of copies. Sometimes, however, this is not the case. When the ratio of the expression levels is not 1 to 1, we call it “allelic imbalance”. There are a variety of reasons why the expression may vary between the alleles. “Gene imprinting,” when environmental factors silence either the maternal or paternal allele, is one case. If one allele is silenced completely, then there will be an extreme case of allelic imbalance. Other scenarios may increase or decrease expression of one particular allele only slightly, resulting in imbalance to a lesser degree. Cis-acting mutations may alter regulation for just one allele through a change to promoter/enhancer regions (transcription factor binding sites), or even through 3′ UTR mutations that affect mRNA stability or microRNA binding.