A system of diagnostic test methods are provided for diagnosing whether a primary breast tumor from an individual human subject is a clinically metastatic tumor. These in-vitro diagnostic methods detect and utilize the presence or absence of a 55 kilodalton protein for the DNA or the RNA coding for an expressing this protein as a marker and indicator of tumor metastasis. The test methods thus can detect either the presence of the protein itself, or its unique DNA, or its singular RNA individually or collectively. Each method of detection provides a reliable indicator and marker by which to clinically diagnose and determine if a primary tumor within the breast of a living human subject is now or will soon likely be a metastatic tumor.