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Positional cloning has shown that the Huntington disease (HD) mutation is an expanded trinucleotide repeat in the IT15 gene. Although this mutation clearly produces the HD phenotype, the function of the Huntington disease protein remains undefined. One recent immunocytochemical study suggested that the IT15 protein preferentially localizes to the nucleus of affected neuronal cells. If this result is accurate, it could link the biochemical function of this protein to nuclear activities such as gene regulation. To examine the nuclear transport of the Huntington disease protein, we searched for basic peptide motifs that could produce nuclear localization. One peptide (RRKGKEK) was identified that is highly homologous to a consensus nuclear localization signal. When fused to the cytoplasmic reporter protein, beta-galactosidase, nuclear localization was observed in stably transformed human cell lines. In a complementary study, an anti-peptide polyclonal antibody, raised against a sequence adjacent to the putative nuclear localization sequence, detected the IT15 protein in the nucleus of human cells. These results extend and confirm the previous localization studies and identify an IT15 peptide motif that can function for nuclear localization.