tr-.alpha.IIb, a soluble, truncated integrin found to be exclusively expressed in tumor cells is provided. An additional truncated integrin, tr-.beta.3, has also been found to be exclusively expressed in tumor cells. Diagnostic compositions including nucleic acid probes and antibodies and methods for detecting the presence of tr-.alpha.IIb and tr-.beta.3 to identify the presence of tumor cells in a sample are also provided.