Genomic imprinting refers to a recently discovered phenomenon in which
the expression pattern of an allele depends on whether that allele was
inherited from the mother or the father. This difference in expression
strategy correlates with differences in the epigenetic state of the two
alleles. These epigenetic differences include DNA methylation at CpG
dinucleotides, as well as modifications on the histones associated with
the locus. In the simplest possible cases, the promoter region of the
imprinted gene is methylated during oogenesis, but not spermatogenesis
(or vice versa). This methylation (and its accompanying histone
modifications) results in inactivation of the modified allele. Of
course, most imprinted genes do not fall into this simplest case. The
goal of this book is neither to provide a basic introduction to
imprinting, nor to provide a comprehensive survey of the current state
of the field (which would necessarily span multiple books). Rather, the
book covers on some of the more recent advances, with the goal of
drawing attention to some of the emerging subtleties and complexities
associated with imprinted genes.