Three clones for chicken chromobox proteins were obtained from liver and ovary cDNA libraries. pCHCB1 and pCHCB2 encode polypeptides showing 96 and 95% identity with mouse M31 and M32, respectively, which are homologues of Drosophila heterochromatin protein 1 (HP1), and pCHCB3 encodes a polypeptide whose sequences of chromobox and C-terminal region show high-level similarities with those of mouse M33, Drosophila polycomb (Pc) protein, and Xenopus Pc homologue. When these cDNAs were expressed in female chicken embryonic fibroblasts (CEFs) as GFP-fused or HA-tagged proteins, all three proteins were found to be localized in nuclei. Among them, CHCB1 associates with brightly stained spots with 4', 6-diamidino-2-phenylindole (DAPI), suggesting its accumulation on heterochromatins. One of those spots was identified as W-heterochromatin. When CHCB1 lacking the N-terminal basic/acidic region or a part of the chromobox region was overexpressed in CEFs, W-heterochromatin became partially or extensively decondensed in the majority of nuclei. Overexpression of CHCB3 lacking a part of the chromobox did not cause decondensation of W-heterochromatin. Specific antisera raised against a part of CHCB1 or CHCB2, produced in Escherichia coli, detected protein species having apparent molecular masses of 25 kDa or 22 plus 23 kDa, respectively, in the subnuclear fraction containing the majority of chromatin from female chicken MSB-1 cells.