From Wikipedia:
Polycomb-group proteins (PcG proteins) are a family of protein complexes first discovered in fruit flies that can remodel chromatin such that epigenetic silencing of genes takes place. Polycomb-group proteins are well known for silencing Hox genes through modulation of chromatin structure during embryonic development in fruit flies (Drosophila melanogaster). They derive their name from the fact that the first sign of a decrease in PcG function is often a homeotic transformation of posterior legs towards anterior legs, which have a characteristic comb-like set of bristles.
In Cognimed: Polycomb is the reader of the inherited program. In our model, PRC2 reads the zygote’s methylation record in reverse developmental order, and its clocked withdrawal is what unlocks each cell fate at the proper time. See Cognimed — Cognitive Biology.