ROBO1 has been implicated as a tumor suppressor for various types of cancer, including breast cancer, and the transmembrane proline-rich γ-carboxyglutamic acid protein 4 (PRRG4) may be a novel target for treating metastatic breast cancer; however, the regulation of ROBO1 expression during tumorigenesis remains poorly understood. This evidence concerns the gene PRRG4 and cancer.