The amount of steroid hormone receptor proteins does not always correlate with the response of breast cancers to endocrine therapy. This may partly be due to the fact that binding of the estrogen receptor (ER) to estrogen responsive elements (ERE) of its target genes is mediated by additional cellular proteins. One of these is the high mobility group protein HMGB1, known to interact with ER thus dramatically increasing its binding to ERE. This is the first report analysing the expression patterns of HMGB1 in breast cancer cells. Northern blot analyses of the 1.4 kb and the 2.4 kb transcripts of HMGB1 in 13 breast cancer samples revealed a strong intertumoural variation by a factor of 8.5 and 14.5, respectively. This variation may contribute to the different response, of estrogen receptor-positive breast tumours to endocrine therapy, making HMGB1 a marker of considerable clinical interest.