ZR-75-1 (a) and BT-474 (b) tumor xenografts excised from female BALB/c (nu/nu) nude mice after four-week treatments with trastuzumab and fulvestrant as single agents or in combination. Days versus tumor volume curves for ZR-75-1 (c) and BT-474 (d) tumor xenografts show that trastuzumab and fulvestrant as single agents or in combination significantly inhibited tumor growth (P<0.05) and that tumor growth in the combination group was notably much more inhibited compared with that in individually treated groups (P<0.05).