In order to infect a cell, the envelope glycoprotein GP120 of the HIV virus interacts with CD4 (acting as the primary receptor) and a co-receptor: either CCR5 or CXCR4. This binding results in membrane fusion and the subsequent intracellular signaling that facilitates viral invasion. In approximately half of all HIV cases, the viruses using the CCR5 co-receptor seem to favor immediate infection and transmission while those using the CXCR4 receptor do not present until later in the immunologically suppressed stage of the disease. The virus will often switch from using CCR5 to CXCR4 during the course of the infection, which serves as an indicator for the progression of the disease. Recent evidence suggests that some forms of HIV also use the large integrin a4b7 receptor to facilitate increased binding efficiency in mucosal tissues.