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We have isolated by differential RNA display a cDNA that is up-regulated in metastatic mammary tumor lines. This cDNA corresponds to HR6B, the yeast homologue of Rad6, a ubiquitin-conjugating enzyme, and a key player in postreplication repair and induced mutagenesis in the yeast. We show that Rad6 protein expressed in metastatic tumor lines is wild type and functional, because it is able to catalyze the transfer of ubiquitin to histone H2b and is predominantly localized in the nucleus as compared with cytoplasmic localization in normal or nonmetastatic mammary cells. This pattern of Rad6 protein expression/localization is not restricted to breast cancer cell lines, because human breast carcinomas display similar patterns of Rad6 up-regulation and nuclear localization suggesting that deregulation in expression of Rad6 may be an important step in transformation to malignant phenotype. Constitutive …