A new method of differential expression cloning called differential display (DD) has been used to screen for novel tumor suppressor genes involved in breast cancer. The screen is based on positive selection at the mRNA level for genes expressed in normal mammary epithelial cells but decreased or lost in corresponding tumor cells. A candidate tumor suppressor gene recovered by DD is integrin alpha-6 (alpha 6), a component of the heterodimeric integrin receptors alpha 6 beta 1 and alpha 6 beta 4. Loss of alpha 6 expression was confirmed in total RNAs by Northern blot analysis and by immunostaining with alpha 6 antibodies. Consistent with these cell culture findings, previous immunostaining of mammary tissue sections has identified decreased alpha 6 protein expression during breast tumor progression. Southern blot analysis demonstrated that alpha 6 gene is present in tumor cell lines, suggesting that reexpression may be inducible by pharmacological intervention. The likelihood that alpha 6 may have tumor suppressing activity is supported by growing evidence of a central role for integrins in transducing growth control and differentiation signals from growth factors and the extracellular matrix (ECM).