ING1 has regulatory roles in the expression of genes associated with proliferation, apoptosis, and senescence. p33(ING1b) is the most widely expressed isoform of the gene. Downregulation of its nuclear expression is involved in differentiation and pathogenesis in invasive breast carcinoma. Yet the mechanism(s) by which p33 nuclear targeting is regulated remains unknown. In this study, we analyzed human invasive breast carcinoma tissue samples by immunostaining with p33 and correlating p33 location with the presence of ERa. Our findings show the expression of p33 protein in ER alpha-positive tumor samples was in the nucleus alone, while the expression was mainly in the cytoplasm in ERa-negative tumor samples. Examination of the localization of p33 in the nucleus and/or cytoplasm in several different cell lines demonstrated 17 beta-estradiol (E2) treatment causes dramatic compartmental shift in p33 protein from the cytoplasm to the nucleus in ER alpha-positive MDA-66 cells. No significant differences in ER alpha-negative MDA-MB-231 cells in the same conditions were observed. We show for the first time nuclear localization of p33 is regulated by estradiol induction in ER alpha-positive breast cancer cells. These results suggest compartmental shift in p33 by ER signaling may be an important molecular event in the differentiation and pathogenesis of invasive breast cancer.