Monoallelic expression from biallelic genes is frequently observed in diploid eukaryotic organisms. Classic examples of this phenomenon include the well-characterized cases of genomic imprinting and X-chromosome inactivation. However, recent studies have shown that monoallelic expression is widespread in autosomal genes. This discovery was met with great interest because it represents another mechanism to generate diversity in gene expression that can affect cell fate and physiology. To date, the molecular mechanisms underlying this phenomenon are largely unknown. In our original study describing the dominant/recessive relationships of pollen-determinant alleles in Brassica self-incompatibility, we found that the recessive allele was specifically methylated and silenced through the action of small RNA derived from the dominant allele. In this review, we focus on recent studies of monoallelic expression in autosomal genes, and discuss the possible mechanisms driving this form of monoallelic gene suppression.