The first step of the HIV replication cycle is attachment and fusion. During this stage, the envelope glycoprotein gp120 on the surface of the virus binds specifically to the CD4 receptor on host immune cells, such as T-cells, macrophages, monocytes, and dendritic cells. The gp120 protein then binds to a smaller coreceptor, either CCR5 or CXCR4, to initiate fusion of the viral envelope with the host cell membrane.