SAM domains (also known as Pointed, HLH, or SPM domains) are ~70 residues long and have been reported as common protein-protein interaction modules [15-17]. This domain is found in a large number of proteins, including Polycomb group (PcG) proteins [18], serine threonine kinases [19], Eph family receptor tyrosine kinases [20], the p73 tumor suppressor [21], the RNA-binding protein Smaug [22], diacylglycerol kinases [23,24], yeast mating type signaling proteins [19,25] and ETS family transcription factors [26,27]. The PcG proteins are transcriptional repressors that maintain gene silencing during development [28-30]. In mammals, PcG proteins are also implicated in Hox gene regulation. Their biological activity lies in stable silencing of specific sets of genes through chromatin modifications. A member of polycomb repressive complex 1 (PRC1), ph, contains a SAM domain at the C-terminal, and PRC1 complex is known to form helical, head-to-tail polymers through its SAM domain [31]. These polymeric structures mediate the formation of a higher order chromatin structure and possibly stabilize the repressed state of Hox genes.