Patients with easily accessible tumors (generally, metastatic deposits involving skin, subcutaneous tissues, or peripheral lymph node whose excision would not require general anaesthesia or the invasion of a body cavity) at the screening visit for participation in Study CP-MGA271-01 (the main study) will be asked to participate in this substudy. After providing appropriate informed consent, patients will undergo excisional biopsy, punch biopsy, or core needle biopsy of the accessible tumor prior to receiving study drug in the main study and then again after the first cycle of study drug is completed.