Although several genetic events, such as changes in ST6GALNAC5, CXCR4/CXCL12, and Slit2/Robo1, have been sporadically reported to correlate with breast cancer brain metastasis (BCBM) in the past few decades,7, 8, 9 the mechanisms underlying the central nervous system (CNS) relapse of breast cancer remain largely unknown. The gene discussed is CXCL12; the disease is breast cancer.