Modifiers of hetero-chromatin. (PEV) and members of the Polycomb-group (Pc-group) gencs en-code proteins that are thought to modify chromatin, rendering it heritably changed in its expressibility (Gaunt and Singh 1990; Paro 1990). Isolation and se-quencing of the PC-group namesake, Polycomb (Pc), has revealed a region of homology with a modifier of PEY known as heterochromatin protein 1 (HPI) (Paro and Hogness 1991). We have shown that the region of homology shared by HPl and Pc, termcd chromo box, can be used to detect homologous sequences in a wide range of animal and plant species (Singh et al. 1991). In addition, this conserved motif has been used to clone the first chromobox-containing genes from mouse (M31 and M32) and human (HSMJ). We have argued that the mammalian HPl-like genes might fonction as gene repressors in a manner similar to their fly ho-molog, perhaps by serving as components of hetero-chromatin.