RCAS1 also known as EBAG9 is an estrogen-regulated protein that plays a critical role in immune evasion of tumor cells. RCAS1 is primarily located on the cell membrane and in soluble forms, where RCAS1 interacts with a putative receptor on peripheral lymphocytes, thereby inhibiting T lymphocyte growth and promoting apoptosis. RCAS1 presence in various cancers, including uterine, ovarian, and lung cancers, highlights RCAS1 significance in tumor progression and immune escape mechanisms. High levels of RCAS1 expression are associated with advanced tumor stages and poor prognoses in cancer patients, making RCAS1 a potential biomarker for assessing tumor aggressiveness.