Drosophila Polycomb group proteins are thought to form multimeric nuclear complexes that are responsible for stable transmission of repressed states of gene expression during the proliferation of differentiating embryos. In this study, we cloned, sequenced, and characterized two Polycomb group homologs, designated pcl and psel, in zebrafish. Amino acid sequence analyses determined that pcl is a structural homolog of Drosophila Polycomb and that pscl is a homolog of Drosophila Posterior sex combs. Northern blots and whole-mount in situ hybridization revealed that pcl and pscl had overlapping expression patterns at successive stages of embryogenesis. Immunocytochemistry localized both Pcl and Pscl protein in blastomere nuclei. Pull-down assays and two-hybrid system deletion analyses showed that these proteins were capable of homotypic and heterotypic interactions and identified the regions required for these interactions. The evidence supports the idea that zebrafish Polycomb group proteins, like those of other species, form nuclear complexes with compositions that may vary in a spatio-temporal manner during development. (C) 2002 Elsevier Science (USA). All rights reserved.