This animation shows the spread of cancer via blood vessels. In this case, the final destination is guided by signaling molecules that bind to the migrating cells. In the example shown, the signaling molecules are SDF-1 and the receptor is CXCR4. This is thought to occur in breast cancer and some other cancer types. CXCR4, (a CXC chemokine Receptor), also called fusin, is an alpha-chemokine receptor specific for stromal-derived-factor-1 (SDF-1 also called CXCL12), a molecule endowed with potent chemotactic activity for lymphocytes. This receptor is one of several chemokine receptors that HIV isolates can use to infect CD4+ T cells. Traditionally, HIV isolates that use CXCR4 are known as T-cell tropic isolates. Typically these viruses are found late in infection. It is unclear whether the emergence of CXCR4-using HIV is a consequence or a cause of immunodeficiency.