The transcriptional repressing Polycomb-group (PcG) and transcriptional activating trithorax-group (trxG) genes of Drosophila are part of a cellular memory system responsible for the stable inheritance of gene activity. PcG proteins assemble into multimeric protein complexes, which are involved in maintaining the transcriptional repressive state of genes over successive cell generations. EED (embryonic ectoderm development) is the human homolog of Eed, a murine PcG gene homologous to the Drosophila homeotic gene, extra sex combs. The human EED protein is 99. 5% identical to the mouse EED protein and contains seven WD repeats, which are involved in protein-protein interactions. There are two human EED transcripts that contain a putative 407-nucleotide-long intron and give rise to two HEED protein isoforms, 535 and 494 amino acids in length. EED interacts in a highly specific manner, both in vitro and in vivo, with histone deacetylase (HDAC) proteins.