Conrad Waddington coined the term epigenetics as “the
branch of biology which studies the causal interactions
between genes and their products, which bring the phenotype
into being” (Waddington, 1942). More recently, Holliday
(1990) described epigenetics as the temporal and spatial
control of gene activity during development of complex
organisms. Finally, Riggs and Porter (1996) modified the
concept to the mitotically heritable changes in gene expression
that occur without changes in DNA sequences. Some
authors emphasize that the term epigenetics implies that the
changes are transmitted to the offspring. In this review, we
refer to epigenetics as the changes in gene expression that
occur without modification in DNA sequences that can be
passed along by mitosis.