Imprinting: a gamete's point of view.

The recent isolation of imprinted mammalian genes has at last allowed the analysis of the molecular controls that regulate monoparental gene expression. Remarkably, many expectations as to how the imprinting mechanism works have been confounded by these studies. As a result, the time now seems right to reconsider our current understanding of the nature of imprinting, and how best it can be defined. Here, I discuss the past and present understanding of imprinting in the light of results obtained from studies of endogenous imprinted genes, and finally propose that the current multiplicity of imprinting terms and definitions be replaced by a single term and definition.