This study is for patients with a type of cancer called carcinoma of unknown primary site (CUP), meaning that the site of the body where the tumor has originated is not clear. Currently, carcinoma of unknown primary site (CUP) accounts for about 5% of all newly diagnosed malignancies. The stage of the tumor is such that it is not treatable by surgery. Currently, the standard treatment for such a type of cancer at that stage is chemotherapy. However, the overall prognosis for patients with metastatic CUP remains poor, even if treated with conventional chemotherapy. Through ongoing research at medical centers around the world, doctors are trying to improve on the presently available chemotherapy regimens. The purpose of the investigators' study is similar: it is trying to determine whether a combination of three chemotherapy drugs - carboplatin, gemcitabine, and capecitabine - will improve the treatment of patients with metastatic CUP. The reason the investigators are interested in the above combination of chemotherapy agents is that each one of them is already used in patients with a variety of specific tumors, such as lung cancer, breast cancer, pancreatic cancer, colon cancer, etc.