Polycomb repression controls the expression of hundreds of genes involved in 
development and is mediated by essentially two classes of chromatin-associated 
protein complexes. The Polycomb repressive complex 2 (PRC2) trimethylates 
histone H3 at lysine 27, an epigenetic mark that serves as a docking site for 
the PRC1 protein complex. Drosophila core PRC1 is composed of four subunits: 
Polycomb (Pc), Posterior sex combs (Psc), Polyhomeotic (Ph) and Sex combs extra 
(Sce). Each of these proteins has multiple orthologs in vertebrates, thus 
generating an enormous scope for potential combinatorial diversity. In 
particular, mammalian genomes encode five Pc family members: CBX2, CBX4, CBX6, 
CBX7 and CBX8. To complicate matters further, distinct isoforms might arise from 
single genes. Here, we address the functional role of the two human CBX2 
isoforms. Owing to different polyadenylation sites and alternative splicing 
events, the human CBX2 locus produces two transcripts: a 5-exon transcript that 
encodes the 532-amino acid CBX2-1 isoform that contains the conserved 
chromodomain and Pc box and a 4-exon transcript encoding a shorter isoform, 
CBX2-2, lacking the Pc box but still possessing a chromodomain. Using 
biochemical approaches and a novel in vivo imaging assay, we show that the short 
CBX2-2 isoform lacking the Pc box, does not participate in PRC1 protein 
complexes, but self-associates in vivo and forms complexes of high molecular 
weight. Furthermore, the CBX2 short isoform is still able to repress 
transcription, suggesting that Polycomb repression might occur in the absence of 
PRC1 formation.