A fundamental problem in biology concerns how the genomic information present in fertilized eggs can give rise to the full spectrum of stably differentiated cell types required to form vertebrates and invertebrates. In the 1930s, C.H. Waddington’s largely observational mammalian embryology studies, which defined this problem, were central to establishing the field of epigenetics. It is now well known that there are master regulatory genes that must be kept on to specify a given cell lineage and off in the many other cell lineages that make up the body.
The problem of keeping these genes in the off state when required has received considerable attention, in large part due to the landmark genetic studies initiated by Pam and Ed Lewis in the 1940s that identified a set of genes required for this repression. This family of genes is called the Polycomb Group (PcG) because the visual phenotype of a heterozygous ...
