Despite the overexpression of HER2 in up to 61% of USC, 38% of clear cell, and 11% of grade 3 endometrioid adenocarcinoma of the uterus found in large cooperative Gynecologic Oncology Group studies (i.e., GOG 177 and GOG 181B) 20,21 as well as encouraging case reports on a limited number of patients 22–24, single agent, trastuzumab 4 mg/kg in week 1 then 2 mg/kg weekly until disease progression in stage III/IV or recurrent endometrial cancers at the phase II level failed to demonstrate significant activity (GOG-181B) 21. This evidence concerns the gene ERBB2 and endometrioid adenocarcinoma.