In contrast, this relationship is generally not seen in ERα-positive/ERβ-positive breast tumours (Jarvinen et al, 2000; Mann et al, 2001; Omoto et al, 2001; Murphy et al, 2002; Fuqua et al, 2003; Iwase et al, 2003; Fleming et al, 2004; Hopp et al, 2004; Myers et al, 2004; Nakopoulou et al, 2004) and therefore our data together with two other studies support the conclusion that the role of ERβ1 when expressed alone in human breast cancers in vivo is likely quite different to when it is coexpressed with ERα. This evidence concerns the gene ESR2 and breast cancer.