The Drosophila gene Polycomb (Pc) has been implicated in the clonal inheritance of determined states and is a trans-regulator of the Antennapedia-like homeobox genes. Pc shares a region of homology (the chromobox) with the Drosophila gene Heterochromatin Protein 1 (HP1), a component of heterochromatin. The Pc chromobox has been used to isolate a mouse chromobox gene, M33, which encodes a predicted 519 amino acid protein. The M33 chromodomain is more similar to that in the Pc protein, than that in the HP1 protein. In addition to the chromodomain, the M33 and Pc proteins also share a region of homology at their C termini. The temporal and spatial expression patterns of M33 have been studied by in situ hybridization and northern analysis. During the final 10 days of embryonic development, M33 expression mirrors that of the cell-cycle-specific cyclin B gene. It is therefore suggested that the rate of cellular proliferation controls M33 expression. From comparisons of the characteristics of M33 with those of Pc it is proposed that M33 is a Pc-like chromobox gene. The roles of M33 and Pc in models of cellular memory are examined and implications of the memory models addressed.