In murine fibroblasts, C3H10T1/2, and human breast cancer cells, MDA-MB-361, where high levels of expression of erbB2 (naturally overexpressed) and erbB3 (ectopically or naturally overexpressed) are evident, mutual interaction between erbB2 and erbB3 takes place. This evidence concerns the gene ERBB3 and breast carcinoma.