Estrogen receptor–binding fragment-associated antigen 9 (EBAG9) gene was identified as an estrogen-responsive gene. The gene product, receptor-binding cancer antigen expressed on SiSo cells (RCAS1), is associated with aggressive characteristics and poor overall survival for 15 different human malignancies. The correlation between RCAS1 expression and several clinicopathological variables, including tumor size, clinical stage, invasion depth and lymph node metastasis highlights this molecule's clinical significance. Expression of RCAS1 in tumor cells plays an important role in evasion from host immune system resulting tumor progression, invasion and metastasis. Further exploration of RCAS1 biological function will facilitate development of novel therapeutic strategies that target RCAS1.