Retinoic acid receptor beta 2 (RARbeta2) isoform has been considered a putative tumor suppressor because it is expressed in normal cells but is lacking in most tumors, including breast cancer. Recently, we identified a novel RARbeta isoform (beta5) in breast cancer cells, which may sereve as a potential target of retinoids in cancer prevention and therapy studies. In this review are summarized the data on the expression of RARbeta5 and of the previously identified RARbeta4 and RARbeta' isoforms in various breast cancer cell lines. We found that RARbeta5 may serve as a potential biomarker of resistance of breast cancer cells to retinoids and thus may have clinical implication in selecting patients that may benefit the most from clinical trials with retinoids.