The present invention relates to a polypeptide derived from a highly conserved region (HCR) I-III of an extracellular region of a CD99 and CD99 family such as CD99L2 and PBDX(or XG), which are a kind of transmembrane protein, or a fused protein thereof. The polypeptide or the fused protein thereof has an activating function of inhibiting the extravasation of white blood cells, or inhibiting the growth and/or metastasis of cancer cells. The present invention also provides a polynucleotide coding the polypeptide, a vector including same, and a transformant transformed by the vector. In addition, the present invention provides a pharmaceutical composition including the polypeptide or the fused protein thereof for preventing or treating inflammatory diseases. Further, the present invention provides a is pharmaceutical composition including the polypeptide or the fused protein thereof inhibiting the growth and/or metastasis of cancer cells, i.e., a pharmaceutical composition for preventing or treating cancer.