Even though angiogenesis is a hallmark of cancer development, some endogenous proteins, such as thrombospondin (Dawson et al. 1997), endostatin (Boehm et al. 1997), and soluble human leukocyte antigen-G (sHLA-G) (Fons et al. 2006), could counteract microvessel formation. Here, HLA-G is linked to cancer.