An isolated sequence SGSSEEKQNAVSSEET (OPNcPEP) SEQ ID NO: 8, and uses thereof. The peptide enhanced soft agar clone formation but did not support the growth of cells in plastic dishes, consistent with supporting anchorage-independence rather than growth. This sequence represented and is unique for a domain around the splice junction of OPN variant -c (OPN-c). OPN-c was expressed in a variety of tumor cell lines, but not in normal tissues (e.g., non-cancerous tissue) or in benign tumors. OPN-c antibody may be administered to a patient with a cancer associated with OPN-c expression to prevent the formation and growth of metastases. OPN-c may be used as a diagnostic to determine whether a patient has a malignant, rather than a benign, growth. OPN-c may be used to detect or identify agents that inhibit or mimic OPN-c expression or activity.