CXCR4 and leukemia: In the beginning, CXCR4 was recognized as a co-receptor in human immunodeficiency virus-1 (HIV-1), which attracted the researchers’ attention; afterwards, more investigation revealed overexpression of CXCR4 in 30 different cancers, including pancreatic, breast, lung, colorectal, prostate, ovarian, and skin cancers, lymphoma, and leukemia [101,102].