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Heterochromatin protein 1 plays an important role in chromatin structure and gene expression regulation. Three <i>HP1</i> genes have been found in <i>Homo sapiens</i>, and five <i>HP1</i> genes have been reported in <i>Drosophila melanogaster</i>. On the other hand, in <i>Bombyx mori</i>, only two <i>HP1</i> genes, <i>BmHP1a</i> and <i>BmHP1b</i>, were reported. In this research, we have reported the molecular and functional characterization of a novel <i>Bombyx mori HP1</i> gene (<i>BmHP1c</i>), which had stronger transcriptional repression activity than BmHP1a. BmHP1a and BmHP1b is reported to form homo- and heterodimers, but in co-immunoprecipitation experiments, no homo- or hetero-dimer formation of BmHP1c with the other silkworm HP1s is detected. The intracellular localization of BmHP1c is not only in the nucleus but also in the cytoplasm like mammalian HP1<i>γ</i>. In contrast to human HP1<i>a</i> and b, all three BmHP1s were localized preferentially in the regions poorly stained with DAPI. Interestingly, the double knockdown of <i>BmHP1a</i> and <i>b</i>, but not <i>BmHP1c</i> with <i>a</i> or <i>b</i>, arrested the cell cycle at the G2/M phase. These results suggest that BmHP1c is not essential for cell progression and plays a different role than BmHP1a and BmHP1b.